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Secrets  of  Practical 
Cement  Construction 


PRICE  $1.50 


The  Cement  Institute  • » 708  N.  7th  St. 

SAINT  - - LOUIS.  - - MISSOURI 


Copyriglif  1906 
by 

Fred.  Eckhard 
Saint  Louis 


= INDEX  = = 


^Le.^  % 


Basement  Floors 

Beam  and  Floor  Slabs 
Bridge  Construction 
Bridge — One  Span  Creek 
Bridge — Flat  Span  Highway 
Building  Blocks 
Building  Block  Machines 

Appearance  and  Use 
City  Specifications 
— Concrete  for 
— Cost  of 
Efflorescence 
— Facing  of 
Hardening 
Materials 
Mixing 

Standard  Specifications 
■—“Strength  of 
— Tamping — how  to 
Use  of  Rich  Mixtures 
Facing 

Waterproof  Qualities 

Colored  Mortar  Table 

Column  Construction 

Fence  Posts  .... 

Freezing— How  to  Prevent 
Measuring  for  Mixing 
Mixing  • 

Re-Inforced  Concrete  Building 
Re-inforced  Designing  Table 


6 

32 

37 

37 

39 

- 10  to  28 

17 
24 
27 
11 
22 

18 
21 
18 

15&16 

16 

10 

19 

17  & 18 

20 
21 

- 1 to  3 

45 

34 

9 

1 

1 

1 

28  to  37 
46  to  47 


« 


■■  f— < 

5 JUJO 


INDEX  = Concluded 


Sidewalks 

— Excavation 
— Sub-Foundation 
— Top  Dressing 
Silos  of  Concrete 
Stable  Floors  and  Driveways 
Steps 

Structural  Details 
Water-proofing 

Building  Blocks 


3 to 


36 
33 
1 to  3 
3 


Miscell; 


Illustrations- 


Hydrated  Lime — How  to  make 

2 

Milk  of  Lime 

3 

Properly  Graded  Materials 

2 

Barn 

43 

Beam  and  Floor  Slabs 

32  & 34 

Columns 

34 

Creek  Bridge 

37 

Facing  Plate 

43 

Flat  Span  Bridge 

39 

Gateway 

44 

Re-Inforced  Building 

29 

Re-Inforcement  for  Bridges 

41 

Sidewalk  Construction 

- 4 

Silo  Construction 

11 

Stirups — Fig.  4 

- 29 

Steps  - 

36 

Structural  Details 

33 

CD  CO  tO  OO  N 


MEASURING. 


Measure  all  parts  in  a barrel  with  the  bottom  out  is  a convenient 
way  to  measure  sand  gravel  or  crushed  rock.  Four  bags  of  cement 
equals  a barrel  of  cement.  In  estimating  do  not  make  the  mistake  so 
often  made  thinking  that  five  barrels  crushed  stone  (or  gravel),  three 
barrels  sand  and  one  barrel  (4  bags)  cement  will  make  ten  barrels  con- 
crete, as  the  sand  and  cement  do  not  take  up  any  space,  but  simply  fills 
in  between  the  broken  stone  (or  gravel),  unless  one-half  or  three-fourths 
rough  stuff  is  used. 

MIXING. 

A watertight  mixing  board  should  be  used,  made  of  one-inch  lumber, 
well  cleated  at  bottom,  with  a 3x4  scantling  around  the  outside  of  top, 
so  as  to  prevent  the  mixture  from  leaving  the  mixing  board.  First  place 
the  sand  on  the  board,  then  the  cement,  then  mix  the  sand  and  cement 
thoroughly  until  the  mass  is  of  an  even  color,  then  wet  and  mix  thor- 
oughly, then  add  crushed  rock,  or  gravel.  This  method  is  for  course 
rock,  or  gravel,  say  about  2-inch  stuff.  One-half  or  three-quarter  inch 
rock  or  gravel  should  be  mixed  with  the  sand  and  cement  in  the  dry 
state  all  at  one  time,  and  then  add  water.  This  will  save  labor.  Do  not 
make  the  concrete  sloppy,  have  it  just  wet  enough  that  when  well 
tramped  the  water  will  come  to  the  surface.  A sprinkling  can  is  a good 
way  to  add  water,  as  it  will  not  wash  away  the  cement.  Do  not  use  a 
hose  until  you  have  become  experienced. 

FREEZING. 

The  best  method  to  keep  cement  work  from  freezing  is  to  warm  the 
sand  and  stone  and  use  hot  water  to  mix.  This  will  make  the  cement 
set  quickly.  Frost  will  not  injure  cement  after  it  has  set,  but  avoid 
cement  work  in  cold  weather  if  possible,  as  frost  will  prevent  the  bond- 
ing of  the  different  layers  and  cause  the  outside  to  scale.  Another 
method  is  to  use  about  ten  pounds  of  salt  to  a barrel  of  cement. 

WATERPROOFING. 

If  it  is  desired  to  make  a leaner  concrete  the  following  methods  of 
waterproofing  may  be  used  with  success.  Hydrated  lime  slightly  delays 
the  setting  of  the  cement  and  it  will  effervesce  to  an  extent, -but  the 
ultimate  formation  of  the  carbonate  of  lime  closes  the  pores  in  the  con- 
crete and  makes  it  impermeable  permanently. 


1 


HYDRATED  LIME. 


Place  the  lime  in  a shallow  box  exposed  to  the  air  as  much  as  pos-  . 
sible,  but  protected  from  rain,  sprinkle  with  a sprinkling  can  just  a little 
every  day,  so  as  to  cause  the  lime  to  fall  to  dust  (hydrated  lime).,  Care 
must  be  taken  not  to  use  too  much  water,  so  as  to  cause  the  lime  to  cook. 
(See  page  for  manner  of  mixing  hydrated  lime  with  cement.) 

WATERPROOF  QUALITIES. 

The  chief  fault  of  concrete  building  blocks,  as  ordinarily  made,  is 
their  tendency  to  absorb  water.  In  this  respect  they  are  generally  no 
worse  than  sandstone  or  common  brick;  it  is  well  known  that  stone  or 
brick  walls  are  too  permeable  to  allow  plastering  directly  on  the  inside 
surface,  and  must  be  furred  and  lathed  before  plastering,  to  avoid 
dampness.  This  practice  is  generally  followed  with  concrete  blocks, 
but  their  use  and  popularity  would  be  greatly  increased  if  they  were  made 
sufficiently  waterproof  to  allow  plastering  directly  on  the  inside  surface. 

For  this  purpose  it  is  not  necessary  that  blocks  should  be  perfectly 
waterproof,  but  only  that  the  absorption  of  water  shall  be  slow,  so  that 
it  may  penetrate  only  part  way  through  the  wall  during  a long-continued 
rain.  Walls  'made  entirely  water-tight,  are,  in  fact,  objectionable,  owing 
to  their  tendency  to  “sweat’*  from  condensation  of  moisture  on  the  in- 
side surface.  For  health  and  comfort  walls  must  be  slightly  porous,  so 
that  any  moisture  formed  on  the  inside  may  be  gradually  absorbed  and 
carried  away. 

Excessive  water  absorption  may  be  avoided  in  the  following  ways: 

1. — Use  of  Properly  Graded  Materials. — It  has  been  shown  that  po- 
rosity and  permeability  are  two  different  things;  porosity  is  the  total 
proportion  of  voids  or  open  spaces  in  the  mass,  while  permeability  is  the 
rate  at  which  water,  under  a given  pressure,  will  pass  through  it.  Per- 
meability depends  on  the  size  of  the  openings  as  well  as  on  their  total 
amount.  In  two  masses  of  the  porosity  or  percentage  of  voids,  one  con- 
sisting of  coarse  and  the  other  of  fine  particles,  the  permeability  will  be 
greater  in  case  of  the  coarse  material.  The  least  permeability,  and  also  the 
least  porosity,  are,  however,  obtained  by  use  of  a suitable  mixture  of 
coarse  ^nd  fine  particles.  Properly  graded  gravel  or  screenings,  containing 
plenty  of  coarse  fragments  and  also  enough  fine  material  to  fill  up  the 
pores,  will  be  found  to  give  a much  less  permeable  concrete  than  fine  or 
coarse  sand  used  alone.. 


2 


MILK  OF  LIME 


Used  in  place  of  clear  water  in  mixing  facing  for  cement  blocks. 
Mix  lime  and  water  as  you  would  whitewash.  This  will  lighten  the 
color  of  your  block  as  well  as  make  them  waterproof. 

United  States  government  engineers  have  used  this  process  in  forti- 
fication work.  One  specification  prepares  a’  wash  of  one  pound  con- 
centrated lye  and  five  pounds  of  alum  in  two  quarts  of  water  to  one 
part,  which  is  added  to  ten  pounds  of  cement,  light  colored  preferred. 
The  wash  should  be  applied  on  a bright  day,  as  the  sun  will  bleach  the 
wash,  making  the  work  a very  light  co.lor.  Wet  the  wall  before  applying 
this  wash. 

For  cistern  work  the  usual  wash  is  simply  cement  grout,  applied 
with  a brush  in  one  or  two  coats,  usually  to  a well  trowled  surface. 

FOR  CEMENT  BUILDING  BLOCKS. 

Take  5 per  cent  solution  of  ground  alum  in  watej*  and  a 7 per  cent 
solution  of  common  yellow  soap  and  water.  Use  the  alum  solution  in 
mixing  mortar  half  as  much  as  the.  usual"  percentage  of' water,  then  add 
the  other  half  in  the  form  of  the  soap  solution.  Use  this  for  the  facing- 
mixture  only.  Twenty  pounds  of  alum  to  a barrel  of  water  and  five  bars 
of  soap  to  a barrel  of  water  makes  a good  solution. 

When  a concrete  block  building  has  already  been  erected,  a wash 
composed  of  barium  hydrate  5 ounces  to  each  gallon  of  water  may  be 
used.  Apply  to  the  surface  of  the  wall.  Several  coats  should  be 
applied  at  intervals.  The  solution  must  be  used  fresh,  as  it  soon  be- 
comes turbid  if  left  in  the  air.  This  solution  is  cheap  and  effective.  It 
fills  and  seals  the  pours  by  absorbing  carbonic  acid  from  the  air. 

CONCRETE  SIDEWALKS. 

A useful  and  comparatively  simple  application  of  concrete  is  in  the 
construction  of  sidewalks,  for  which  .purpose  it  has  been  used  with 
marked  success  for  a number  of  years. 

EXCAVATION  AND  PREPARATION  OF  SUBGRADE. 

The  ground  is  excavated  to  subgjade  and  well  consolidated  by  ram- 
ming to  prepare  it  for  the  subfoundation  of  stone,  gravel  or  cinders.  The 
depth  of  excavation  will  depend  upon  the  climate  and  nature  of  the 
ground,  being  deeper  in  localities  where  heavy  frosts  occur,  or  where 


the  ground  is  soft,  than  in  climates  where  there  are  no  frosts.  In  the 
former  case  the  excavation  should  be  carried  to  a depth  of  12  inches, 
whereas  in  the  latter  from  4 to  6 inches  will  be  sufficient.  No  roots  of 
trees  should  be  left  above  subgrade.  } 


THE  SUBFOUNDATION. 

The  subfoundation  consists  of  a layer  of  loose  material,  such  as 
broken  stone,  gravel  or  cinders.  Spread  over  the  subgrade  and  well 
tamped  to  secure  a firm  base  for  the  main  foundation  of  concrete,  which 
is  placed  on  top.  It  is  most  important  that  the  subfoundation  be  well 
drained  to  prevent  the  accumulation  of  water,  which,  upon  freezing, 
would  lift  and  crack  the  walk.  For  this  purpose  it  is  well  to  provide 
drain  tile  at  suitable  points  to  carry  off  any  water  which  may  collect 
under  the  concrete.  An  average  thickness  for  subfoundation  is  4 to  6 
inches.  Although  in  warm  climates,  if  the  ground  is  firm  and  well' 
drained,  the  subfoundation  may  be  only  2 to  3 inches  thick,  or  omitted 
altogether. 


THE  FOUNDATION. 

The  foundation  consists  of  a layer  of  concrete  deposited  on  the  sub- 
foundation and  carrying  a surface  layer,  or  wearing  coat,  of  cement 
mortar.  If  the  ground  is  firm  and  the  subfoundation  well  rammed  in 


4 


place  and  properly  drained,  great  strength  will  not  be  required  of  the 
concrete,  which  may,  in  such  cases,  be  mixed  in  about  the  proportions 
1-3-6,  and  a depth  of  only  3 to  4 inches  will  be  required.  Portland 
cement  should  be  used  and  stone  or  gravel  under  1 inch  in  size,  the  con- 
crete being  mixed  of  medium  consistency,  so  that  moisture  will  show 
on  the  surface  without  excessive  tamping. 


THE  TOP  DRESSING  OR  WEARING  SURFACE. 

To  give  a neat  appearance  to  the  finished  walk,  a top  dressing  of 
cement  mortar  is  spread  over  the  concrete,  well  wo'rked  in,  and  bro  ught 
to  a perfectly  smooth  surface  with  straight  edge  and  float.  This  mortar 
should  be  mixed  in  the  proportion  of  1 part  cement  to  2 parts  sand, 
sharp,  coarse  sand  or  screenings  below  one-fourth  inch  of  some  hard, 
tough  rock  being  used.  The  practice  of  making  the 'concrete  of  natural 
cement  and  the  wearing  surface  of  Portland  is  not  to  be  commended, 
owing  to  a tendency  for  the  two  to  separate. 

A cord  stretched  between  stakes  will  serve  as  a guide  in  excavating, 
after  which  the  bottom  of  the  trench  is  well  consolidated  by  ramming, 
any  loose  material  below  subgrade  being  replaced  by  sand  or  gravel. 
The  material  to  form  the  subgrade  is  then  spread  over  the  bottom  of 
the  trench  to  the  desired  thickness  and  thoroughly  compacted.  Next, 
stakes  are  driven  along  the  sides  of  the  walk,  spaced  4 to  6 feet  apart, 
and  their  tops  made  even  with  the  finished  surface  of  the  walk,  which 
should  have  a transverse  slope  of  one-fourth  inch  to  the  foot  for  drain- 
age. Wooden  strips  at  least  1^2  inch  thick  and  of  suitable  depth  are 
nailed  to  these  stakes  to  serve^  as  a mold  for  the  concrete.  By  carefully 
adjusting  these  strips  to  the  exact  height  of  the  stakes  they  may  be  used 
as  guides  for  the  straightedge  in  leveling  off  the  concrete  and  wearing 
surface.  The  subfoundation  is  well  sprinkled  to  receive  the  concrete, 
which  is  deposited  in  the  usual  manner,  well  tapnped  behind  a board  set 
vertically  across  the  trench,  and  leveled  off  with  a straightedge,  as 
shown,  leaving  one-half  to  one  inch  for  the  wearing  surface.  Three- 
eighths  inch  sand  joints  are  provided  at  intervals  of  6 to  8 feet,  to  pre- 
vent expansion  cracks,  or,  in  case  of  settlement,  to  confine  the  cracks 
to  these  joints.  This  is  done  either  by  depositing  the  concrete  in  sec- 
tions, or  by  dividing  it  into  such  sections  with  a spade  when  soft  and 
filling  the  joints  with  sand.  The  location  of  each  joint  is  marked  on 
the  wooden  frame  for  future  reference.  Care  must  be  exercised  to  pre- 
vent sand  or  any  other  material  from  being  dropped  on  the  concrete, 


5 


and  thus  preventing  a proper  union  with  the  wearing  surface.  No  sec- 
tion should  be  left  partially  completed  to  be  finished  with  the  next  batch 
or  left  until  the  following  day.  Any  concrete  left  after  the  completion 
of  a section  should  be  mixed  with  the  next  batch.  It  is  of  the  utmost 
importance  to  follow  up  closely  tl;e  concrete  work  with  the  top  dress- 
ing in  order  that  the  two  may  set  together.  This  top  dressing  should  be 
worked  well  over  the  concrete  with  a trowel,  and  leveled  with  a straight- 
edge, to  secure  an  even  surface.  Upon  the  thoroughness  of  this  opera- 
tion often  depends  the  success  or  failure  of  the  walk,  since  a good  bond 
between  the  wearing  surface  and  concrete  base  is  absolutely  essential. 
The  mortar  should  be  mixed  rather  stiff.  As  soon  as  the  film  of  water 
begins  to  leave  the  surface,  a wooden  float  is  used,  followed  up  by  a 
plasterer’s  trowel,  the  operation  being  similar  to  that  of  plastering  a 
wall.  The  floating,  though  necessary  to  give  a smooth  surface,  will,  if 
continued  too  long,  brings  a thin  layer  of  neat  cement  to  the  surface  and 
probably  cause  the  walk,  to  crack.  The  surface  is  now  divided  into  sec- 
tions by  cutting  entirely  through,  exactly  over  the  joints  in  the  concrete. 
This  is  done  with  a trowel  guided  by  a straightedge,  after  which  the 
edges  are  rounded  off  with  a special  tool  called  a jointer,  having  a thin 
s-hallow  tongue.  These  sections  may  be  subdivided  in  any  manner  de- 
sired for  the  sake  of  appearance.  A special  tool  called  an  edger  is  run 
around  the  outside  of  the  walk  next  to  the  mold,  giving  it  a neat  rounded 
edge.  A toothed  roller  having  small  projectigns  on  its  face  is  frequently 
used  to  produce  slight  indentations  on  the  surface,  adding  somewhat  to 
the  appearance  of  the  walk.  The  completed  work  must  be  protected 
from  the  sun  and  kept  moist  by  sprinkling  for  several  days.  In  freezing 
weather  the  same,  precautions  should  be  taken  as  in  other  classes  of 
concrete  work. 

CONCRETE  BASEMENT  FLOORS. 

Basement  floors  in  dwelling  houses  as  a rule  require  only  a moderate 
degree  of  strength,  although  in  cases  of  very  wet  basements,  where 
water  pressure  from  beneath  has  to  be  resisted,  greater  strength  is  re- 
quired than  would  otherwise  be  necessary.  The  subfoundation  should 
be  well  drained,  sometimes  requiring  the  use  of  tile  for  carrying  off  the 
water.  The  rules  given  for  constructing  concrete  sidewalks  apply  equally 
well  to  basement  floors.  The  thickness  of  the  concrete  foundation  is 
usually  from  3 to  5 inches,  according  to  strength  desired,  and  for  average 
work  a 1-3-6  mixture  is  sufficiently  rich.  Expansion  joints  are  frequently 
omitted,  since  the  temperature  variation  is  less  than  in  outside  work,  but 
since  this  omission 'not  infrequently  gives  rise  to  unsightly  cracks,  their 
use  is  recommended  in  all  cases.  It  will  usually  be  sufficient  to  divide 


6 


a room  of  moderate  size  into  four  equal  sections,  separated  by  one-half 
inch  sand  joints.  The  floor  should  be  given  a slight  slope  toward  the 
center,  or  one  corner,  with  provision  at  the  lowest  point  for  carrying 
off  any  water  that  may  accumulate. 

CONCRETE  STABLE  FLOORS  AND  DRIVEWAYS. 

Concrete  stable  floors  and  driveways  are  constructed  in  the  same 
general  way  as  basement  floors  and  sidewalks,  but  with  a thicker  foun- 
dation, on  account  of  the  greater  strength  required.  The  foundation 
: : may  well  be  6 inches  thick,  with  a 1 inch  wearing  surface.  An 

objection'  sometimes  raised  against  concrete  driveways  is  that  they 
become  slippery  when  wet,  but  this  fault  is  in  a great  measure  overcome 
by  dividing  the  wearing  surface  into  small  squares  about  4 inches  on  the 
side,  by  means  of  triangular  grooves  three-eighths  of  an  inch  deep.  This 
gives  a very  neat  appearance  and  furnishes  a good  foothold  for  horses. 


7 


Reinforced  concrete  silos  may  be  built  Monolithic,  but  hollow  cement 
building  blocks  will  make  the  neatest  and  most  serviceable  silo  and  at 
the  same  cost  Monolithic  building  requires  greater  skill  than  building 
blocks  on  account  of  cracks  caused  by  contraction  and  expansion,  and 
if  too  much  water  is  used  in  mixing  the  concrete  shrinkage  cracks  will 
appear. 

Farmers  are  beginning  to  use  Portland  cement  concrete  for  silos, 
in  which  root  crops  and  green  fodder  is  stored  for  winter  use,  even 
green  grass  may  be  kept  in  these  silos.  Heretofore  silos  have  been 
constructed  of  wood,  brick  or  stone,  all  of  which  concrete  is  fast  super- 
seding. For  small  silos  plain  Monolithic  structures  are  built,  for  larger 
silos  a simple  form  of  reinforced  concrete  is  coming  into  use,  usually 
expanded  metal  or  plain  iron  telegraph  wire  is  the  reinforcing  member. 

A silo  ten  feet  in  diameter  by  fifteen  feet  high  should  have  the 
foundations  carried  down  below  the  frost  line,  the  footing  being  about 
two  feet  wide  at  the  base,  contracting  to  sixteen  inches  at  the  top  of 
the  wall,  upon  which  a wall  ten  inches  thick  will  answer.  Reinforced 
with  sheets  of  expanded  metal  placed  about  one  inch  from  the  outer 
face  of  the  wall  and  reinforced  by  heavy  wire  wound  around  the  ex- 
panded metal  and  spaced  two  inches  from  centers  for  the  first  five  feet 
of  the  structure  for  a circular  tank  or  silo,  a core  is  erected  against 
which  the  concrete  is  deposited,  a ring  or  circular  form  capable  of  being 
raised  from  time  to  time  as  the  concrete  is  tamped  in  place. 

: : : The  outer  circular  form  as  well  as  the  inner  core 

and  the  method  of  supporting  the  outer  form  by  barrels  and  loose  brick 
piers  each  time  it  is  raised.  This  form  is  in  two  pieces,  capable  of  being 
brought  close  together,  in  order  to  diminish  the  thickness  of  the  wall 
which,  being  ten  inches  at  the  base,  may  be  contracted  to  five  inches  at 
the  top.  The  expanded  metal  sheets  should  lap  five  to  six  inches  at  the 
ends.  The  heavier  the  reinforcement  the  thinner  may  the  walls  be  con- 
structed. In  finishing  the  last  ten  inches  of  wall  three-eighth  inch  iron 
bolts  ten  inches  long  should  be  imbedded  in  the  top,  projecting  four 
inches  out  of  the  concrete,  to  be  used  for  anchoring  the  roof,  which  is 
usually  of  wood.  The  mixture  for  the  concrete  may  be  one  cement, 
three  sand,  and  four  gravel.  Where  hand  mixing  is  the  process,  deposit 
the  sand  and  gravel,  over  which  spread  the  cement  and  turn  not  less 
than  three  times  dry,  then  add  water,  just  sufficient  that  when  thor- 
oughly mixed  the  concrete  will  ball  in  the  hand,  deposit  in  place  and 
ram  or  tamp  well.  In  finishing  the  last  ten  inches  of  the  wall  at  the 


8 


top  eight  or  ten  turns  of  wire  should  be  laid  or  wound  around  the 
■expanded  metal  for  additional  strength.  A silo  fifteen  feet  high  with  an 
internal  diameter  will  have  a capacity  of  about  1,179  cubic  feet.  Doors 
or  openings  are  sometimes  left  in  the  silo  walls  for  convenience  of 
filling  or  removing  the  contents.  For  this  purpose  frame  should  be 
inserted,  as  is  done  in  ordinary  brick  building  construction,  and  the 
doors  hung  after  the  structure  is  finished-  Concrete  silos  when  prop- 
erly built  are  practically  indestructible. 


FENCE  POSTS. 

Fence  posts  should  be  stronger  than  building  blocks,  as  a greater 
strain  is  put  to  them,  for  they  often  receive  sudden  jolts;  various  de- 
vices have  been  invented  for  attaching  fence  wire  to  the  post,  the  con- 
cern you  buy  your  mold  from  will  advise  you  which  is  the  best  adapted 
to  their  particular  make.  A concrete  not  weaker  than  four  parts  of 
coarse,  sharp  sand  to  one  part  of  Portland  cement  should  not  be  used 
fbr  posts. 

Although  a post  may  be  hard  and  apparently  strong  when  one  week 
old,  it  will  not  attain  its  full  strength  in  that  length  of  time  and  must  be 
handled  with  the  utmost  care  to  prevent  injury.  Carelessness  in  hand- 
ling green  posts  frequently  results  in  the  formation  of  fine  cracks,  which, 
though  unnoticed  at  the  time,  give  evidence  of  their  presence  later  in 
the  failure  of  the  post.  Posts  should  be  allowed  to  cure  for  at  least 
sixty  days  before  being  placed  in  the  ground,  and  for  this  purpose  it  is 
recommended  that  when  moved  from  the  molding  platform  they  be 
placed  upon  a smooth  bed  of  moist  sand  and  protected  from  the  sun 
until  thoroughly  cured.  During  this  period  they  should  receive  a thor- 
ough drenching  at  least  once  a day.  The  life  of  the  molds  will  depend 
upon  the  care  with  which  they  are  handled.  A coating  of  mineral  oil  or 
shellac  may  be  used  instead  of  soap -to  prevent  the  cement  from  stick- 
ing to  the  forms.  As  soon  as  the  molds  are  removed  they  should  be 
cleaned  with  a wire  brush  before  being  used  again.  The  cost  of  rein- 
forced concrete  fence  posts  depends  in  each  case  upon  the  cost  of  labor 
and  materials,  and  must  necessarily  vary  in  different  localities.  An 
estimate  in  any  particular  case  can  be  made  as  follows:  One  cubic  yard 

of  concrete  will  make  20  posts,  measuring  6 inches  by  6 inches  at  bot- 
tom, 6 inches  by  3 inches  at  top,  and  7 feet  long,  and  if  mixed  in  the 


proportions  \-2]/2-S,  requires  approximately. 

Materials  for  1 cubic  yard  of  concrete: 

1 barrel  of  cement. 

3 barrels  of  sand. 

5 barrels  of  gravel. 

To  this  must  be  added  the  cost  of  mixing  concrete,  molding  and 
handling  posts. 

STANDARD  SPECIFICATIONS  FOR  CEMENT  BLOCKS. 

One  of  the  most  important  subjects  to  be  considered  by  the.  cement 
block  industry  is  standard  specifications  for  cement  building  blocks 
that  will  meet  the  'approval  of  municipal  corporations,  engineers,  archi- 
tects and  the  private  consumer.  Such  specifications  will  inspire  confi- 
dence and  result  in  the  real  advancement  of  the  industry.  A block  with 
a mixture  of  1 cement  to  4 sand  and  gravel  will  meet  all  of  the  require- 
ments for  strictly  first-class  material.  It  will  be  dense,  fairly  waterproof 
and  sound  in  every  way,  provided  the  curing  is  properly  done.  While 
a weaker  mixture,  say  1 to  7,  will  be  amply  strong  where  the  aggregates 
are  properly  graded,  there  is  yet  an  element  of  danger  in  new  and  inex- 
perienced men,  the  richer  the  mixture  in  cement  the  sooner  will  the  block 
be  ready  to  set  in  the  wall,  thus  a saving  of  . time  in  completing  the  work. 

Blocks  in  which  cement  has  been  skimped  has  now  and  then  re- 
sulted in  giving  the  industry  a black ’eye.  It  is  this  identical  bad  work 
that  has  made  the  insurance  companies  timid  in  giving  reasonable  rates. 
Bad  news  travels  fast.  So  does  bad  work.  In  every  community  there 
are  men  who  are  ever  ready  to  pick  flaws  in  concrete  construction, 
especially  where  the  cement  block  is  displacing  wood,  stone  and  brick 
the  cement  block  is  therefore  on  trial  and  will  be  compared  with  other 
forms  and  materials  of  construction.  It  behooves  the  cement  block 
manufacturer  not  to  skimp  the  materials,  but  always  to  turn  out  strictly 
high  grade  work,  and  he  will  be  the  gainer  in  the  end  and  overcome 
the  kickers  who  are  constantly  crying  down  the  innovation  the  cement 
block  is  effecting  in  the  building  industry.  This  is  particularly  true  of 
a certain  class  of  old  line  architects  who  still  cling  with  Chinese  persist- 
ence to  the  materials  of  their  fathers,  shutting  their  eyes  to  the  living, 
breathing  present  with  its  potent  array  of  force  which  are  constantly 
undermining  and  transforming  old  ideas. 

Unfortunately,  owing  to  poor  workmanship  and  lack  of  artistic  de- 
sign, a large  part  of  the  hollow  block  buildings  hitherto  erected  have 
fallen  far  short  of  the  excellence  above  described.  A multitude  of  men 


10 


without  capital  and  inexperienced  in  the  use  of  cement  have  embarked 
in  the  business  of  block  making,  attracted  by  the  glowing  prospects  of 
profits  held  out  by  the  army  of  block  machine  agents.  As  a result, 
great  quantities  of  inferior  blocks,  weak,  porous  and  unsound,  have  been 
and  are  being  turned  out,  and  have  been  erected  by  careless  and  unskilled 
builders  into  defective  and  ugly  structures.  This  state  of  affairs  is  an 
injury  to  competent  and  conscientious  block  manufacturers,  and  an 
obstacle  to  the  adoption  of  a most  excellent  and  promising  building 
material.  Blocks  of  first-rate  quality  can  easily  and  cheaply  be  made, 
with  small  outlay  for  machinery,  provided  certain  simple  rules  are  intelli- 
gently followed.  It  is  the  purpose  of  this  paper  to  state  briefly  the 
causes  of  faults  in  concrete  blocks,  and  the  precautions  by  which  good 
and  reliable  work  may  be  assured. 


CONCRETE. 

Concrete  is  an  artificial  stone  consisting  of  coarse  and  fine  fragments, 
such  as  sand,  gravel  and  broken  stone,  united  by  cement  to  a solid  mass. 

The  strength  of  concrete  depends  greatly  upon  its  density,  and  this 
is  secured  by  using  coarse  material  which  contains  the  smallest  amount 
of  voids  or  empty  spaces.  Different  kinds  of  sand,  gravel  and  stone 
vary  greatly  in  the  amount  of  voids  they  contain,  and  by  judiciously 
mixing  coarse  and  fine  material  the  voids  may  be  much  reduced  and  the 
density  increased.  The  density  and  percentage  of  voids  in  concrete  ma- 
terial may  be  determined  by  filling  a box  of  one  cubic  foot  capacity  and 
weighing  it.  One  cubic  foot  of  solid  quartz  or  limestone,  entirely  free 
from  voids,  would  weigh  165  pounds,  and  the  amount  by  which  a cubic 
foot  of  any  loose  material  falls  short  of  this  weight  represents  the  pro- 
portion of  voids  contained  in  it.  For  example,  if  a cubic  foot  of  sand 
weighs  115j/2  pounds,  the  voids  would  be  49j4-165ths  of  the  total  volume, 
or  30  per  cent. 

Experiments  have  shown  that  the  strength  of  concrete  increases 
greatly  with  its  density;  in  fact,  a slight  increase  in  weight  per  cubic 
foot  adds  very  decidedly  to  the  strength. 

The  gain  in  strength  obtained  by  adding  coarse  material  to  mixtures 
of  cement  and  sand-is  shown  in  the  following  table  of  results  of  experi- 
ments made  in  Germany  by  R.  Dykerhoff.  The  blocks  tested  were  214- 
inch  cubes,  1 day  in  air  and  27  days  in  water: 


1 

2 



33 

2,125 

1 

2 

5 

12.5 

2,387 

1 

3 

— 

25 

1,383 

1 

3 

9.5 

1,515 

1 

4 

20 

1,053 

1 

4 

8^4 

7.4 

1,204 

11 


These  figures  show  how  greatly  the  strength  is  improved  by  adding- 
coarse  material,  even  though  the  proportion  of  cement  is  thereby  re- 
duced. A mixture  of  1 to  \2]/2  of  properly  proportioned  sand  and  gravel 
is,  in  fact,  stronger  than  1 to  4,  and  nearly  as  strong  as  1 to  3,  of  cement 
and  sand  only. 

In  selecting  materials  for  concrete,  those  should  be  chosen  whicu 
give  the  greatest  density.  If  it  is  practicable  to  mix  two  materials,  as 
sand  and  gravel,  the  proportion  which  gives  the  greatest  density  should 
be  determined  by  experiment,  and  rigidly  adhered  to  in  making  concrete, 
whatever  proportion  of  cement  it  is  decided  to  use.  Well  proportioned 
dry  sand  and  gravel  or  sand  and  broken  stone,  well  shaken  down, -should 
weigh  at  least  125  pounds  per  cubic  foot.  Limestone  screenings,  owing 
to  minute  pores  in  the  stone  itself,  are  somewhat  lighter,  though  giving 
equally  strong  concrete.  They  should  weigh  at  least  120  pounds  per  cubic 
foot.  If  the  weight  is  less,  there  is  probably  too  much  fine  dust  in  the 
mixture. 

The  density  and  strength  of  concrete  are  also  greatly  improved  by 
use  of  a liberal  amount  of  water.  Enough  water  must  be  used  to  make 
the  concrete  thoroughly  soft  and  plastic,  so  as  to  quake  strongly  when 
rammed.  If  mixed  too  dry  it  will  never  harden  properly,  and  will  be 
light,  porous  and  crumbling. 

Thorough  mixing  of  concrete  materials  is  essential,  to  increase  the 
density  and  give  the  cement  used  a chance  to  produce  its  full  strength. 
The  cement,  sand  and  gravel  should  be  intimately  mixed,  dry,  then  the 
water  added  and  the  mixing  continued.  If  stone  or  coarse  gravel  is 
added,  this  should  be  well  wetted  and  thoroughly  mixed  with  the  mortar. 


MATERIALS  FOR  CONCRETE  BUILDING  BLOCKS. 

In  the  making  of  building  blocks  the  spaces  to  be  'filled  with  con- 
crete are  generally  too  narrow  to  permit  the  use  of  very  coarse  material, 
and  the  block-maker  is  limited  to  gravel  or  stone  not  exceeding  y2  or 
Y inch  in  size.  A considerable  proportion  of  coarse  material  is,  how- 
ever, just  as  necessary  as  in  other  kinds  of  concrete  work,  and  gravel 
or  screenings  should  be  chosen  which  will  give  the  greatest  possible 
density.  For  good  results,  at  least  one-third  of  the  material,  by  weight, 
should,  be  coarser  than  % inch.  Blocks  made  from  such  gravel  or 
screenings,  1 to  5,  will  be  found  as  good  as  1 to  3 with  sand  only.  It  is 
a mistake  to  suppose  that  the  coarse  fragments  will  show  on  the  surface; 


12 


if  the  mixing  is  thorough  this  will  not  be  the  case.  A moderate  degree 
of  roughness  or  variety  in  the  surface  of  blocks  is,  in  fact,  desirable,  and 
would  go  far  to  overcome  the  prejudice  which  many  architects  hold 
against  the  smooth,  lifeless  surface  of  cement  work. 

Sand  and  gravel  are,  in  most  cases,  the  cheapest  material  to  use  for 
block  work.  The  presence  of  a few  per  cent,  of  clay  or  loam  is  not 
harmful  provided  the  mixing  is  thorough. 

Stone  screenings,  if  of  good  quality,  give  fully  as  strong  concrete  as 
sand  and  gravel,  and  usually  yield  blocks  of  somewhat  lighter  color. 
Screenings  from  soft  stone  should  be  avoided,  also  such  as  contain  too 
much  dust.  This  can  be  determined  from  the  weight  per  cubic  foot,  and 
by  a'  sifting  test.  If  more  than  two-thirds  pass  j^-inch,  and  the  weight 
(well  jarred  down)  is  less  than  120  pounds,  the  material  is  not  the  best. 

Cinders  are  sometimes  used  for  block  work;  they  vary  greatly  in 
quality,  but  if  clean  and  of  medium  coarseness  will  give  fair  results. 
Cinder  concrete  never  develops  great  ‘ strength,  owing  to  the  porous 
character  and  crushability  of  the  cinders  themselves.  Cinder  blocks  may, 
however,  be  strong  enough  for  many  purposes,  and  suitable  for  work  in 
which  great  strength  is  not  required. 

Lime. — It  is  well  known  that  slaked  lime  is  a valuable  addition  to 
cement  mortar,  especially  for  use  in  air.  In  sand  mixtures,  1 to  4 qr  1 
to  5,  at  least  one-third  of  the  cement  may  be  replaced  by  slaked  lime 
without  loss  of  strength.  The  most  convenient  form  of  lime  for  use  in 
block-making  is  the  dry-slaked  or  hydrate  lime,  now  a common  article 
of  commerce.  This  is,  however,  about  as  expensive  as  Portland  cement, 
and  there  is  no  great  saving  in  its  use.  Added  to  block  concrete,  in  the 
proportion  of  % to  V2  the  cement  used,  it  will  be  found  to  make  the 
blocks  lighter  in  color,  denser,  and  decidedly  less  permeable  by  water. 

Cement. — Portland  cement,  today,  is  the  only  hydraulic  material  to 
be  seriously  considered  by  the  block-maker,  and  at  present  prices  there 
is  nothing  gained  by  attempting  the  use  of  any  of  the  cheaper  substitutes. 
Natural  and  slag  cements  and  hydraulic  lime  are  useful  for  work  which 
remains  constantly  wet,  but  greatly  inferior  in  strength  and  durability 
when  exposed  to  dry  air.  A further  advantage  of  Portland  cement  is 
the  promptness  with  which  it  hardens  and  develops  its  full  strength;  this 
quality  alone  is  sufficient  to  put  all  other  cements  out  of  consideration 
for  block  work. 


13 


PROPORTIONS. 


There  are  three  important  considerations  which  must  be  kept  in  view 
in  adjusting  the  proportions  of  materials  for  block  concrete — strength, 
permeability,  and  cost. 

So  far  as  strength  goes,  it  may  easily  be  shown  that  concretes  very 
poor  in  cement,  as  1 to  8 or  1 to  10,  will  have  a crushing  resistance  far 
beyond  any  load  that  they  may  be  called  upon  to  sustain.  Such  con- 
cretes are,  however,  extremely  porous,  and  absorb  water  like  a sponge. 
It  is  necessary,  also,  that  the  blocks  sh'all  bear  a certain  amount  of  rough 
handling  at  the  factory  and  while  being  carted  to  work  and  set  up  in 
the  wall,  and  safety  in  this  respect  calls  for  a much  greater  degree  of 
hardness  than  would  be  needed  to  bear  the  weight  of  the  building.  Again, 
strength  and  hardness,  with  a given  proportion  of  cement,  depend  greatly 
on  the  character  of  the  other  materials  used;  blocks  made  of  cement  and. 
sand,  1 to  3,  will  not  be  so  strong  or  so  impermeable  to  water  as  those 
made  from  a good  mixed  sand  and  gravel,  1 to  5.  On  the  whole,  it  is 
doubtful  whether  blocks  of  satisfactory  quality  can  be  made,  by  hand 
mixing  and  tamping,  under  ordinary  factory  conditions,  from  a poorer 
mixture  than  1 to  5.  Even  this  proportion  requires  for  good  results  the 
use  of  properly  graded  sand  and  gravel  or  screenings,  a liberal  amount  of 
water,  and  thorough  mixing  and  tamping.  When  suitable  gravel  is  not 
obtainable,  and  coarse  mixed  sand  only  is  used,  the  proportion  should 
not  be  less  than  1 to  4.  Fine  sand  alone  is  a very  bad  material,  and  good 
blocks  cannot  be  made  from  it  except  by  the  use  of  an  amount  of  cement 
which  would  make  the  cost  very  high. 

The  mixture  above  recommended,  1 to  4 and  1 to  5,  will  necessarily 
be  somewhat  porous,  and  may  be  decidedly  so  if  the  gravel  or  screenings 
used  is  not  properly  graded.  The  water-resisting  qualities  may  be  greatly 
improved,  without  loss  of  strength,  by  replacing  a part  of  the  cement  by 
hydrate  lime.  This  is  a light,  extremely  fine  material,  and  a given  weight 
of  it  goes  much  further  than  the  same  amount  of  cement  in  filling  the 
pores  of  the  concrete.  It  has  also  the  effect  of  making  the  wet  mixture 
more  plastic  and  more  easily  compacted  by  ramming,  and  gives  the  fin- 
ished blocks  a lighter  color. 

The  following  mixtures,  then,  are  to  be  recommended  for  concrete 
blocks.  By  “gravel”  is  meant  a suitable  mixture  of  sand  and  gravel,  or 
stone  screenings,  containing  grains  of  all  sizes,  from  fine  to  j4-inch: 

1 to  4 Mixtures,  by  Weight. 

Cement  150,  gravel  600. 

Cement  125,  Hyd.  lime  25,  gravel  600. 

Cement  100,  Hyd.  lime  50,  gravel  600. 


14 


1 to  5 Mixtures,  by  Weight. 

Cement  120,  gravel  600. 

Cement  100,  Hyd.  lime  20,  gravel  600. 

Proportion  of  Water. — This  is  a matter  of  the  utmost  ^consequence, 
and  has  more  effect  on  the  quality  of  the  work  than  is  generally  sup- 
posed. Blocks  made  from  too  dry  concrete  will  always  remain  soft  and 
weak,  no  matter  how  thoroughly  sprinkled  afterwards.  On  the  other 
hand,  if  blocks  are  to  be  removed  from  the  machine  as  soon  as  made, 
too  much  water  will  cause  them  to  stick  to  the  plates  and  sag  out  of 
shape.  It  is  perfectly  possible,  however,  to  give  the  concrete  enough 
water  for  maximum  density  and  first-class  hardening  properties,  and  still 
to  remove  the  blocks  at  once  from  the  mould.  A good  proportion  of 
coarse  material  allows  the  mixture  to  be  made  wetter  without  sticking 
or  sagging.  Use  of  plenty  of  water  vastly  improves  the  strength,  hard- 
ness and  waterproof  qualities  of  blocks,  and  makes  them  decidedly 
lighter  in  color.  The  rule  should  be: 

Use  as  much  water  as  possible  without  causing  the  blocks  to  stick 
to  the  plates  or  to  sag  out  of  shape  on  removing  from  the  machine. 

The  amount  of  water  required  to  produce  this  result  varies  with  the 
materials  used,  but  is  generally  from  8 to  9 per  cent,  of  the  weight  of  the 
dry  mixture.  A practical  block-maker  can  judge  closely  when  the  right 
amount  of  water  has  been  added,  by  squeezing  some  of  -the  mixture  in 
the  hand.  Very  slight  variations  in  proportion  of  water  make  such  a 
marked  difference  in  the  quality  and  color  of  the  blocks  that  the  water, 
when  the  proper  quantity  for  the  materials  used  has  been  determined, 
should  always  be  accurately  measured  out  for  each  batch.  In  this  way 
much  time  is  saved  and  uncertainty  avoided. 

Facing. — Some  block-makers  put  on  a facing  of  richer  and  finer 
mixture,  making  the  body  of  the  block  of  poorer  and  coarser  material. 
As  will  be  explained  later,  the  advantage  of  the  practice  is,  in  most  cases, 
questionable,  but  facings  may  serve  a good  purpose  in  case  a colored  or 
specially  waterproof  surface  is  required.  Facings  are  generally  made  of 
cement  and  sand  or  fine  screenings,  passing  a I^-inch  sieve.  To  get  the 
same  hardness  and  strength  as  a 1 to  5 gravel  mixture,  at  least  as  rich 
a facing  as  1 to  3 will  be  found  necessary.  Probably  1 to  2 will  be  found 
better,  and  if  one-third  the  cement  be  replaced  by  hydrate  lime  the 
waterproof  qualities  and  appearance  of  the  blocks  will  be  improved.  A 
richer  facing  than  1 to  2 is  liable  to  show  greater  shrinkage  than  the 
body  of  the  block,  and  to  adhere  imperfectly  or  develop  hair-cracks  in 
consequence. 


15 


Poured  Work. — The  above  suggestions  on  the  question  of  propor- 
tions of  cement,  sand  and  gravel  for  tamped  blocks  apply  equally  to 
concrete  made  very  wet,  poured  into  the  mould,  and  allowed  to  harden 
a day  or  longer  before  removing.  Castings  in  a sand  mould  are  made 
by  the  use  of  very  liquid  concrete;  sand  and  gravel  settle  out  too  rapidly 
from  such  thin  mixtures,  and  rather  fine  limestone  screenings  are  gen- 
erally used. 


MIXING.  . 

To  get  the  full  benefit  of  the  cement  used  it  is  necessary  that  all  the 
materials  shall  be  very  thoroughly  mixed  together.  The  strength  of  the 
block  as  a whole  will  be  only  as  great  as  that  of  its  weakest  part,  and 
it  is  the  height  of  folly,  after  putting  in  a liberal  measure  of  cement,  to 
so  slight  the  mixing  as  to  get  no  better  result  than  half  as  much  cement, 
properly  mixed,  would  have  given.  The  poor,  shoddy  apd  crumbly 
blocks  turned  out  by  many  small-scale  makers  owe  their  faults  chiefly 
to  careless  mixing  and  use  of  too  little  water,  rather  than  to  too  small 
proportion  of  cement. 

The  materials  should  be  mixed,  dry,  until  the  cement  is  uniformly 
distributed  and  perfectly  mingled  with  the  sand  and  gravel  or  screenings; 
then  the  water  is  to  be  added  and  the  mixing  continued  until  all  parts 
of  the  mass  are  equally  moist  and  every  particle  is  coated  with  the  ce- 
ment paste. 

Concrete  Mixers. — Hand-mixing  is  always  imperfect,  laborious  and 
slow,  and  it  is  impossible  by  this  method  to  secure  the  thorough  stirring 
and  kneading  action  which  a good  mixing  machine  gives.  If  a machine 
taking  5 or  10  horse  power  requires  five  minutes  to  mix  one-third  of  a 
yard  of  concrete,  is  of  course  absurd  to  expect  that  two  men  will  do 
the  same  work  by  hand  in  the  same  time.  And  the  machine  never  gets 
tired  or  shirks  if  not  constantly  urged,  as  it  is  the  nature  of  men  to  do. 
It  is  hard  to  see  how  the  manufacture  of  concrete  blocks  can  be  success- 
fully carried  on  without  a concrete  mixer.  Even  for  a small  business  it 
will  pay  well  in  economy  of  labor  and  excellence  of  work  to  install  such 
a machine,  which  may  be  driven  by  a small  electric  motor  or  gasoline 
engine.  In  wrork  necessarily  so  exact  as  this,  requiring  perfectly  uniform 
mixtures  and  use  of  a constant  percentage  of  water,  batch  mixers,  which 
take  a measured  quantity  of  material,  mix  it,  and  discharge  it,  at  each 
operation,  are  the  only  satisfactory  type,  and  continuous  mixers  are 
unsuitable.  Those  of  the  pug-mill  type,  consisting  of  an  open  trough 
with  revolving  paddles  and  bottom  discharge,  are  positive  and  thorough 


16 


in  their  action,  and  permit  the  whole  operation  to  be  watched  and  con- 
trolled. They  should  be  provided  with  extensible  arms  of  chilled  iron, 
which  can  be  lengthened  as  the  ends  become  worn. 

Concrete  Block  Systems. — For  smaller  and  less  costly  buildings, 
separate  blocks,  made  at  the  factory  and  built  up  into  the  walls  in  the 
same  manner  as  brick  or  blocks  of  stone,  are  simpler,  less  expensive 
and  much  more  rapid  in  construction  than  monolithic  work.  They  also 
avoid  some  of  the  faults  to  which  solid  concrete  work,  unless  skillfully 
done,  is  subject,  such  as  the  formation  of  shrinkage  cracks. 

Tamped  Blocks  From  Semi-Wet  Mixtures. — These  are  practically 
always  made  on  a block-machine,  so  arranged  that  as  soon  as  a block  is 
formed  the  cores  and  side-plates  are  removed  and  the  block  lifted  from 
the  machine.  By  far  the  larger  part  of  the  blocks  on  the  market  are 
made  in  this  way.  Usually  these  are  of  the  one-piece  type,  in  which 
a single  block,  provided  with  hollow  cores,  makes  the  whole  thickness, 
of  the  wall.  Another  plan  is  the  two-piece  system,  in  which  the  face 
and  back  of  the  wall  are  made  up  of  different  blocks,  so  lapping  over 
each  other  as  to  give  a bond  and  hold  the  wall  together.  Blocks  of  the 
two-piece  type  are  generally  formed  in  a hand  or  hydraulic  press. 

Various  shapes  and  sizes  of  blocks  are  commonly  made;  the  build- 
ers of  the  most  popular  machines  have,  however,  adopted  the  standard 
length  of  32  inches  and  height  of  9 inches  for  the  full-sized  block,  with 
thickness  of  8,  10  and  12  inches.  Lengths  of  24,  16  and  8 inches  are  also 
obtained  on  the  same  machines  by  the  use  of  parting  plates  and  suitably 
divided  face  plates;  any  intermediate  lengths  and  any  desired  heights 
may  be  produced  by  simply  adjustments  or  blocking  off. 

Blocks  are  commonly  made  plain,  rock-faced,  tool-faced,  paneled,  and 
of  various  ornamental  patterns.  New  designs  of  face  plates  are  con- 
stantly being  added  by  the  most  progressive  machine-makers.  The  fol- 
lowing illustrations  show  some  of  the  forms  of  blocks  most  commonly 
made: 

Block  Machines. — There  are  many  good  machines  on  the  market, 
most  of  which  are  of  the  same  general  type,  and  differ  only  in  me- 
chanical details.  They  may  be  divided  into  two  classes:  those  with 
vertical  and  those  with  horizontal  face.  In  the  former  the  face  plate 
stands  vertically,  and  the  block  is  sitnply  lifted  from  the  machine  on  its 
base  plate  as  soon  as  tamped.  In  the  other  type  the  face  plate  forms 
the  bottom  of  the  mould;  the  cores  are  withdrawn  horizontally,  and  by 
the  motion  of  a lever  the  block  with  its  face  plate  is  tipped  up  into  a 


17 


vertical  position  for  removal.  In  case  it  is  desired  to  put  a facing  on  the 
blocks,  machines  of  the  horizontal-face  type  are  considered  the  more 
convenient,  though  a facing  may  easily  be  put  on  with  the  vertical-face 
machine  by  the  use  of  a parting  plate. 

Tamping  of  Concrete  Blocks. — This  is  generally  done  by  means  of 
hand-rammers.  Pneumatic  tampers,  operated  by  an  air-compressor,  are 
in  use  at  a few  plants,  apparently  with  considerable  saving  in  time  and, 
labor  and  improvement  in  quality  of  work.  Moulding  concrete  by  pres- 
sure, either  mechanical  or  hydraulic,  is  not  successful  unless  the  pressure 
is  applied  to  the  face  of  a comparatively  thin  layer.  If  compression  of 
thick  layers,  especially  of  small  width,  is  attempted,  the  materials  arch 
and  are  not  compacted  at  any  considerable  depth  from  the  surface. 
Moulding  blocks  by  pressure  is  therefore  practiced  only  in  the  two-piece 
system,  in  which  the  load  is  applied  to  the  surface  of  pieces  of  no  great 
thickness.  Hand  tamping  must  be  conscientious  and  thorough,  or  poor 
work  will  result.  It  is  important  that  the  mould  should  be  filled  a little 
at  a time,  tamping  after  each  addition;  at  least  four  fillings  and  tamp- 
ings  should  be  given  to  each  block.  If  the  mixture  is  wet  enough  no 
noticeable  layers  will  be  formed  by  this  process. 

Hardening  and  Storage. — Triple  decked  cars  to  receive  the  blocks 
from  the  machines  will  be  found  a great  saving  of  labor,  and  are  essen- 
tial in  factories  of  considerable  size.  Blocks  will  generally  require  to  be 
left  on  the  plates  for  at  least.  24  hours,  and  must  then  be  kept  under  roof, 
in  a well-warmed  room,  with  frequent  sprinkling,  for  not  less  than  five 
days  more.  They  may  then  be  piled  up  out  of  doors,  and  in  dry  weather 
should  be  wetted  daily  with  a hose.  Alternate  wetting  and  drying  is 
especially  favorable  for  the  hardening  of  cement,  and  concrete  so  treated 
gains  much  greater  strength  than  if  kept  continuously  in  water  or  dry  air. 

Blocks  should  not  be  used  in  building  until  at  least  four  weeks  from 
the  time  they  are  made.  During  this  period  of  seasoning,  blocks  will 
be  found  to  shrink  at  least  1-16  inch  in  length,  and  if  built  up  in  a wall 
when  freshly  made,  shrinkage  cracks  in  the  joints  or  across  the  blocks 
will  surely  appear. 

Efflorescence,  or  the  appearaace  of  a white  coating  on  the  surfaces, 
sometimes  takes  place  when  blocks  are  repeatedly  saturated  with  water 
and  then  dried  out;  blocks  laid  on  the  ground  are  more  liable  to  show 
this  defect.  It  results  from  diffusion  of  soluble  sulphates  of  lime  and 
alkalies  to  the  surface.  It  tends  to  disappear  in  time,  and  rarely  is  suffi- 
cient in  amount  to  cause  any  complaint. 


18 


PROPERTIES  OF  CONCRETE  BLOCKS. 


Strength. 

In  the  use  of  concrete  blocks  for  the  walls  of  buildings,  the  stress 
to  which  they  are  subjected  is  almost  entirely  one  of  compression.  In 
compressive  strength  well-made  concrete  does  not  differ  greatly  from 
ordinary  building  stone.  It  is  difficult  to  find  reliable  records  of  tests 
of  sand  and  gravel  concrete,  1 to  4 and  1 to  5,  such  as  is  used  in  making 
blocks;  the  following  figures  show  strength  of  concrete  of  approximately 
this  richness,  also  the  average  of  several  samples  each  of  well-known 
building  stones,  as  stated  by  the  authorities  named: 


Limestone,  Bedford,  Ind.  (Ind.  Geo.  Survey) 7,792  lbs. 

Limestone,  Marblehead,  Ohio  (Q.  A.  Gillmore).- 7,393  lbs. 

Sandstone,  N.  Amherst,  Ohio  (Q.  A.  Gillmore). 5,831  lbs. 

Gravel  Concrete,  1:1. 6:2.8,  at  1 yr.  (Candlot) 5,500  lbs. 

Gravel  Concrete,  1:1. 6:3. 7,  at  1 yr.  (Candlot) 5,050  lbs. 

Stone  Concrete,  1:2:4  at  1 yr.  (Boston  El.  R.  R.) 3,904  lbs. 


Actual  tests  of  compression  strength  of  hollow  concrete  blocks  are 
difficult  to  make,  because  it  is  almost  impossible  to  apply  the  load  uni- 
formly over  the  whole  surface,  and  also  because  a block  16  inches  long 
and  8 inches  wide  will  bear  a load  of  150,000  to  200,000  lbs.,  or  more 
than  the  capacity  of  any  but  the  largest  testing  machines.  Three  one- 
quarter  blocks,  8 inches  long,  8 inches  wide  and  9 inches  high,  with  hol- 
low space  equal  to  one-third  of  the  surface,  tested  at  the  Case  School 
of  Science,  showed  strengths  of  1,805,  2,000  and  1,530  lbs.  per  square  inch, 
respectively  when  10  weeks  old. 

Two  blocks6  x 8 9 inches,  22  months  old,  showed  crushing  strength 
of  2,530  and  2,610  lbs.  per  sq.  inch. 

These  blocks  were  made  of  cement  1%,  lime  y2,  sand  and  gravel  6, 
and  were  tamped  from  damp  mixture. 

It  is  probably  safe  to  assume  that  the  minimum  crushing  strength  of 
well-made  blocks,  1 to  5,  is  1,000  lbs.  per  square  inch  at  1 month  and 

2.000  lbs.  at  1 year. 

Now  a block  12  inches  wide  and  24  inches  long  has  a total  surface 
of  228  sq.  inches,  or,  deducting  1-3  for  openings,  a net  area  of  192  inches. 
Such  a block,  9 inches  high,  weighs  130  lbs.  Assuming  a strength  of 

1.000  lbs.  and  a factor  of  safety  of  5,  the  safe  load  would  be  200  lbs. 
per  sq.  inch,  or  200x192=38,400  lbs.  for  the  whole  surface  of  the  block. 
Dividing  this  by  the  weight  of  the  block,  130  lbs.,  we  find  that  295  such 
blocks  could  be  placed  one  upon  another,  making  a total  height  of  wall  of 
222  ft.,  and  still  the  pressure  on  the  lowest  block  would  be  less  than  one- 


19 


fifth  of  what  it  would  actually  bear. 

This  shows  how  greatly  the  strength  of  concrete  blocks  exceeds  any 
demands  that  are  made  upon  it  in  ordinary  building  construction. 

The  safe  load  above  assumed,  200  lbs.,  seems  low  enough  to  guard 
against  any  possible  failure.  In  Taylor  and  Thompson’s  work  on  con- 
crete a safe  load  of  450  lbs.  for  concrete  1 to  2 to  4 is  recommended; 
this  allows  a factor  of  safety  of  S]/2.  On  the  other  hand,  the  Building 
Code  of  the  City  of  Cleveland  permits  concrete  to  be  loaded  only  to  150 
lbs.  per  sq.  inch  and  limits  the  height  of  walls  of  12-inch  blocks  to  44  ft. 
The  pressure  of  such  a wall  would  be  only  40  lbs.  per  square  inch;  ad- 
ding the  weight  of  two  floors  at  25  lbs.  per  sq.  ft.  each,  and  roof  with  snow 
and  wind  pressure,  40  lbs.  per  sq.  ft.,  we  find  that  with  a span  of  25  ft. 
the  total  weight  on  the  lowest  blocks  would  be  only  52  lbs.  per  sq.  inch, 
or  about  one-twentieth  of  their  minimum  compression  strength. 

Blocks  with  openings  equal  to  only  one-third  the  surface,  as  required 
in  many  city  regulations,  are  heavy  to  handle,  especially  for  walls  12 
inches  and  more  in  thickness,  and,  as  the  above  figures  show,  are  enor- 
mously stronger  than  there  is  any  need  of.  Blocks  with  openings  of  50 
per  cent,  would  be  far  more  acceptable  to  the  building  trade,  and  if 
used  in  walls  not  over  44  ft.  high,  with  floors  and  roof  calculated  as 
above  for  25  feet  span,  would  be  loaded  only  to  56  lbs.  per  square  inch 
of  actual  surface.  This  would  give  a factor  of  safety  of  18,  assuming  a 
minimum  compression  strength  of  1,000  lbs. 

There  is  no  doubt  that  blocks  with  one-third  opening  are  inconven- 
iently and  unnecessarily  heavy.  Such  a block,  32  inches  long,  12  inches 
wide,  and  9 inches  high,  has  walls  about  Zl/2  inches  thick,  and  weighs  180 
lbs.  A block  with  50  per  cent,  open  space  would  have  walls  and  parti- 
tions 2 inches  in  thickness,  and  would  weigh  about  130  lbs.  With  proper 
care  in  manufacture,  especially  by  using  as  much  water  as  possible,  blocks 
with  this  thickness  of  walls  may  be  made  thoroughly  strong,  sound  and 
durable.  It  is  certainly  better  for  strength  and  water-resisting  quali- 
ties to  make  thin-walled  blocks  of  rich  mixture,  rather  than  heavy  blocks 
of  poor  and  porous  material. 

2. — Use  of  Rich  Mixtures. — All  concretes  are  somewhat  permeable 
by  water  under  sufficient  pressure.  Mixtures  rich  in  cement  are  of  course 
much  less  permeable  than  poorer  mixtures.  If  the  amount  of  cement 
used  is  more  than  sufficient  to  fill  the  voids  in  the  sand  and  gravel,  a 
very  dense  concrete  is  obtained,  into  which  the  penetration  of  water  is 
extremely  slow.  The  permeability  also  decreases  considerably  with  age, 
owing  to  the  gradual  crystallization  of  the  cement  in  the  pores,  so  that 


20 


concrete  which  is  at  .first  quite  absorbent  may  become  practically  imper- 
meable after  exposure  to  weather  for  a few  weeks  or  months.  There 
appears  to  be  a very  decided  increase  in  permeability  when  the  cement 
is  reduced  below  the  amount  necessery  to  fill  the  voids.  For 
example,  a good  mixed  sand  and  gravel  weighing  123  lbs.  per  cubic  foot, 
and  therefore  containing  25  per  cent,  voids,  will  give  a fairly  impermeable 
concrete  in  mixtures  up  to  1 to  4,  but  with  less  cemoii  will  be  found 
quite  absorbent.  A gravel  with  only  20  per  cent,  voids  would  give  about 
equally  good  results  with  a 1 to  5 mixture;  such  gravel  is,  however, 
rarely  met  with  in  practice.  On  the  other  hand,  the  best  sand,  mixed 
fine  and  coarse,  seldom  contains  less  than  33  per  cent,  voids,  and  con- 
crete made  from  such  material  will  prove  permeable  if  poorer  than  1 to  3. 

Filling  the  voids  with  cement  is  a rather  expensive  method  of  se- 
curing waterproof  qualities,  and  gives  stronger  concretes  than  are 
needed.  The  same  may  be  accomplished  more  cheaply  by  replacing  part 
of  the  cement  by  slaked  lime,  which  is  an  extremely  fine-grained  material, 
and  therefore  very  effective  in  closing  pores.  Hydrate  lime  is  the  most 
convenient  material  to  use,  but  nearly  as  costly  as  Portland  cement  at 
present  prices.  A 1 to  4 mixture  in  which  one-third  the  cement  is  re- 
placed by  hydrate  lime  will  be  found  equal  to  a 1 to  3 mixture  without 
the  lime.  A 1 to  4 concrete  made  from  cement  1,  hydrate  lime  >4,  sand 
and  gravel  6 (by  weight),  will  be  found  fairly  water-tight,  and  much 
superior  in  this  respect  to  one  of  the  same  richness  consisting  of  cement 
1^2,  sand  and  gravel  6. 

3. — Use  of  a Facing. — Penetration  of  water  may  be  effectively  pre- 
vented by  giving  the  blocks  a facing  of  richer  mixture  than  the  body. 
For  the  sake  of  smooth  appearance,  facings  are  generally  made  of  cement 
and  fine  sand,  and  it  is  often  noticed  that  these  do  not  harden  well.  It 
should  be  remembered  that  a 1 to  3 sand  mixture  is  no  stronger  and 
little  if  any  better  in  water  absorption  than  a 1 to  5 mixture  of  well 
graded  sand  and  gravel.  To  secure  good  hardness  and  resistance  to 
moisture  a facing  as  rich  as  1 to  2 should  be  used. 

General  Hints  on  Waterproof  Qualities. — To  obtain  good  water- 
resisting  properties,  the  first  precaution  is  to  make  the  concrete  suffic- 
iently wet.  Dry-tamped  blocks,  even  from  rich  mixture,  will  always  be 
porous  and  absorbent,  while  the  same  mixture  in  plastic  condition  will 
give  blocks  which  are  dense,  strong,  and  water-tight.  The  difference 
in  this  respect  is  shown  by  the  following  tests  of  small  concrete  blocks, 
made  by  the  writer.  The  concrete  used  was  made  of  1 part  cement  and 
5 parts  mixed  fine  and  coarse  sand,  by  weight. 


21 


No.  1. — With  5 per  cent,  water,  rather  dryer  than  ordinary  block 
concrete,  tamped  in  mould. 

No.  2. — With  10  per  cent,  water,  tamped  in  mould. 

No.  3 — With  25  per  cent,  water,  poured  into  a mould  resting  on  a 
flat  surface  of  dry  sand;  after  1 hour  the  surface  was  troweled  smooth; 
mould  not  removed  until  set. 

These  blocks  were  allowed  to  harden  a week  in  moist  air,  then 
dried.  The  weights,  voids,  and  water  absorption  were  as  follows: 
This  method  will  always  show  hair  or  shrinkage  cracks  on  the  face  of 


blocks. 

1 

2 

3 

Damp-tamped. 

Wet-tamped.  Poured. 

Weight  per  cubic  foot,  lbs 

112.2 

125-9 

112.0 

Voids,  calculated,  per  cent,  of 
volume  

. 25.7 

22.9 

12.5 

Water  required  to  fill  voids,  per 
cent,  of  wt  

9.6 

9.4 

12.5 

Water  absorbed  after  2 hours,  per 
cent  of  wt 

. 8.6 

6.4 

10.0 

The  rate  at  which  these  blocks  absorbed  water  was  then 

determined 

by  drying  them  thoroughly,  then  placing  them 

in  a tray 

containing 

water  14  inch  in  depth,  and  weighing  them  at  intervals. 

Water  absorbed 

1 

2 

3 

per  cent,  by  weight.  Dainp-tamped. 

Wet-tamped.  Poured. 

V2  hour  

..  2.0 

0.8 

1.8 

1 “ . 

3.2 

1.0 

2.5 

2 “ 

4.1 

1.4 

3.2 

4 “ 

5.2 

1.9 

3.8 

24 

6.1 

3.0 

7.0 

48  “ 

....  6.4 

4.1 

7.5 

These  figures  show  that  concrete 

which  is 

sufficienttly 

wet  to  be 

thoroughly  plastic  absorbs  water  much 

more  slowly  than  dryer  concrete, 

and  prove  the  importance  of  using  as  much  water  as  possible  in  the 
damp-tamping  process. 

COST. 

The  success  of  the  hollow  concrete  block  industry  depends  to  a 
great  extent  on  cheapness  of  product,  since  it  is  necessary,  in  order  to 
build  up  a large  business,  to  compete  in  price  with  common  brick  and 
rubble  stone.  At  equal  cost,  well-made  blocks  are  certain  to  be  preferred, 
owing  to  their  superiority  in  strength,  convenience,  accurate  dimensions. 


22 


and  appearance.  For  the  outside  walls  of  handsome  buildings,  blocks 
come  into  competition  with  pressed  brick  and  dressed  stone,  which  are, 
of  course,  far  more  costly.  Concrete  blocks  can  be  sold  and  laid  up  at 
a good  profit  at  25  cents  per  cubic  foot  of  wall.  Common  red  brick 
costs  generally  about  12  dollars  per  thousand,  laid.  At  24  to  the  cubic 
foot,  a thousand  brick  are  equal  to  41.7  cu.  ft.  of  wall;  or,  at  $12,  29c.  per 
cu.  ft.  Brick  walls  with  pressed  brick  facing  cost  from  40c.  to  50c.  per 
cubic  foot,  and  dressed  stone  from  $1  to  $1.50  per  foot. 

The  factory  cost  of  concrete  blocks  varies  according  to  the  cost  of 
materials.  Let  us  assume  cement  to  be  $1.50  per  barrel  of  380  lbs.,  and 
sand  and  gravel  25c.  per  ton.  With  a 1 to  4 mixture  1 barrel  cement 
will  make  1,900  lbs.  of  solid  concrete,  or  at  130  lbs.  per  cu.  ft., -14.6  cubic 
feet.  The  cost  of  materials  will  then  be 

Cement,  380  lbs $1.50 

Sand  and  gravel,  1,520  lbs 0.19 

Total  $1.69 

or  11.5c.  per  cu.  ft.  solid  concrete.  Now,  blocks  9 inches  high  and  32 
inches  long  make  2 square  feet  of  face  of  wall,  each.  Blocks  of  this 
height  and  length,  8 inches  thick,  make  1 1-3  cubic  feet  of  wall;  and 
blocks  12  inches  thick  make  2 cubic  feet  of  wall.  From  these  figures  we 
may  calculate  the  cost  of  materials  for  these  blocks,  with  cores  or  open- 
ings equal  to  1-3  or  the  total  volume,  as  follows: 

Per  cu.  ft.  of  block,  1-3  opening  7.7  cts. 

Per  cu.  ft.  of  block,  y2  opening  . 5.8 

Block  8 x 9 x 32  inches,  1-3  opening  10.3 

Block  8 x 9 x 32  inches,  y2  opening  7.7 

Block  12  x 9 x 32  inches,  1-3  opening  15.4 

Block  12  x 9 x 32  inches,  y2  opening  11.6  “ 

Tf  one-third  of  the  cement  is  replaced  by  hydrate  lime  the  quality 
of  the  blocks  will  be  improved,  and  the  cost  of  material  reduced  about 
10  per  cent. 

The  cost  of  labor  required  in  manufacturing,  handling  and  deliver- 
ing blocks  will  vary  with  the  locality  and  the  size  and  equipment  of  fac- 
tory. With  hand-mixing,  3 men  at  average  of  $1.75  each  will  easily  make 
75  8-inch  of  50  12-inch  blocks,  with  1-3  openings,  per  day.  The  labor 
cost  for  these  sizes  of  blocks  will  therefore  be  7c.  and  lO^c.  respecttively. 
At  a factory  equipped  with  power  concrete  mixer  and  cars  for  transport- 
ing blocks,  in  which  a number  of  machines  are  kept  busy,  the  labor  cost 
will  be  considerably  less.  An  extensive  industry  located  in  a large  city 


23 


is,  however,  subject  to  many  expenses  which  are  avoided  in  a small 
country  plant,  such  as  high  wages,  management,  office  rent,  advertising, 
etc.,  so  that  the  total  cost  of  production  is  likely  to  be  about  the  same  in 
both  cases.  A fair  estimate  of  total  factory  cost  is  as  follows: 


Material. 

Labor. 

Total. 

8 

x 32  inch,  1-3  space  

10.3 

7 

17.3  cts. 

8 

x 32  inch,  Yz  “ 

77 

6 

13.7  “ 

12 

x 32  inch,  1-3  “ 

15.4 

10.5 

25.9  “ 

12 

x 32  inch,  “ 

11.6 

9 

20.6  “ 

With  fair  allowance  for  outside  expenses  and  profit,  8-inch  blocks 
may  be  sold  at  30c.  and  12-inch  at  40c.  each.  For  laying  12-in.  blocks  in 
the  wall,  contractors  generally  figure  about  10c.  each.  Adding  5c.  for 
teaming,  the  blocks  will  cost  55c.  each,  erected,  or  27^c.  per  cubic  foot  of 
wall.  This  is  less  than  the  cost  of  common  brick,  and  the  above  fig- 
ures show  that  this  price  could  be  shaded  somewhat,  if  necessary,  to 
meet  competition. 


APPEARANCE  AND  USE. 

Since  concrete  blocks  are,  as  has  been  shown,  more  convenient, 
more  efficient,  and  cheaper  than  any  other  building  material,  it  would 
naturally  be  expected  that  they  would  quickly  take  the  place  of  wood, 
brick  and  stone  and  be  generally  adopted  for  all  ordinary  construction. 
The  growth  of  the  block  industry  has,  indeed,  been  rapid,  but  it  plays 
as  yet  but  a small  part  in  the  building  operations  of  the  country.  It  is 
evident  on  all  sides  that  concrete  blocks  meet  with  opposition  and  sus- 
picion on  the  part  of  architects  and  builders,  and  in  consequence  are  much 
less  generally  adopted  than  their  merits  appear  to  warrant.  * Tt  is  neither 
just  nor  expedient  to  attribute  this  opposition  to  prejudice  against  a 
new  material.  Rather  should  we  try  to  find  and  remove  the  grounds  pn 
which  such  opposition  is  based.  My  observation  leads  me  to  believe 
that  architects  and  engineers  have  no  prejudice  against  concrete,  but  on 
the  contrary,  welcome  it  as  a building  material  by  means  of  which  they 
can  obtain  results  never  before  within  their  reach.  And  they  are  also 
keenly  watching  the  block  industry,  and  are  ready  to  adopt  block  con- 
struction as  soon  as  they  are  offered  a product  which  meets  their  ideas 
as  to  utility  and  beauty. 

Fortunately,  no  material  is  so  elastic  in  its  capabilities  as  concrete, 
and  no  other  can  with  so  little  effort  be  adapted  to  produce  any  effect 


24 


desired.  It  is  hardly  to  be  expected  that  the  block  of  the  present  day 
will  be  the  block  of  the  future;  the  type  which  is  most  economical,  prac- 
tical and  beautiful  will  gradually  come  to  the  front,  and  that  which  is 
costly,  clumsy  add  ugly  will  become  a thing  of  the  past.  To  make  a 
success  of  the  business  we  must  keep  our  eyes  open,  watch  what  others 
are  doing  in  the  way  of  invention  and  improvement,  and  study  the  wants 
of  customers.  And  we  must  not  hesitate  to  throw  our  old  block  machines 
into  the  scrap  heap  when  we  are  sure  we  have  found  a better  apparatus 
and  process. 

The  objections  which  architects  and  builders  make  to  blocks  now  on 
the  market  are  chiefly  the  following: 

Poor  workmanship, 

Fixed  dimensions, 

Too  great  weight, 

Unpleasing  appearance. 

As  to  workmanship,  shoddy,  weak  and  crumbling  blocks  are  far  too 
often  met  with.  Good  concrete  should  be  hard  and  dense, _ and  should 
give  out  a musical  tone  when  struck  with  a hammer.  If  your  blocks 
sound  dead  when  struck,  and  break  easily  with  an  earthy  fracture,  you 
are  either  using  too  poor  a mixture  or  working  too  dry,  probably  the  lat- 
ter. It  does  not  pay,  for  the  sake  of  low  factory  cost,  to  turn  out  work  of 
this  kind.  If  there  is  any  money  to  be  made  in  the  block  business  it 
will  be  made  by  furnishing  a good  article  at  a living  price,  and  in  no 
other  way.  Will  any  one  argue  that  it  pays  to  make  rotten  blocks  at  a 
factory  cost  of  two  cents  less  than  good  ones?  My  belief  is  that  the  ten- 
dency of  the  future  will  be  toward  the  use  of  wetter  concrete,  and  the 
adoption  of  a process  which  makes  this  possible. 

As  to  fixed  dimensions  of  blocks,  the  standard  length  of  32  inches, 
divided  into  halves,  thirds  and  quarters,  is  very  convenient,  and  is  gener- 
ally confirmed  to  by  architects  for  simple  work,  without  much  objection. 
To  be  fully  successful,  however,  and  to  overcome  all  prejudice,  the 
block-maker  must  be  ready  to  furnish  any  size  or  shape  that  may  be 
called  for  to  suit  architects’  designs.  It  would  be  very  pleasant  if  we 
could  confine  ourselves  to  the  standard  size  and  let  customers  “take  it 
or  leave  it.”  But  such  an  attitude  bars  the  way  to  any  wide  use  of 
blocks  in  varied  and  attractive  buildings,  and  cannot  be  maintained  with- 
out loss  of  trade.  Architects  want  also  courses  of  greater  or  less 
height  than  the  9 inch  standard,  and  all  manner  of  cornices,  copings, 
columns  and  capitals.  This  may  frighten  the  timid  and  conservattive 
block-maker,  but  it  is  in  that  direction  that  success  lies,  and  the  pro- 
duction of  these  special  shapes  requires  only  ingenuity,  courage  and  me- 


25 


chanical  skill.  Until  we  can  say  to  the  architect  “Design  whatever  you 
like,  we’ll  make  it  for  you,"  he  will  shy  at  us  and  our  product.  He  will, 
of  course,  readily  appreciate  that  special  shapes  cost  more  than  standard, 
and  if  he  knows  he  can  get  just  what  he  wants  he  will  be  conveniently  and 
cheaply  furnished. 

Preference  should  be  given,  therefore,  to  the  machine  which  per- 
mits the  greatest  variety  of  sizes  and  shapes  to  be  easily  made.  And 
the  greatest  business  success  is  likely  to  come  "to  the  manufacturer  who 
shows  the  least  inclination  to  get  into  a rut,  and  is  most  ready  to  adapt 
his  product  to  the  wants  of  his  patrons. 

The  objection  to  the  weight  of  the  one-piece  block  comes  chiefly  from 
masons  and  contractors.  Hoisting  12  x 32  inch  blocks  weighing  180  lbs. 
to  the  upper  floors  of  a building,  and  handling  them  onto  the  wall,  is  a 
considerable  taslc,  and  it  is  largely  on  this  account  that  the  half-block 
of  the  two-piece  system,  24  inches  long,  weighing  only  64  lbs.,  is  received 
with  so  much  favor.  It  must  be  remembered,  however,  that  the  two- 
piece  blocks  make  a wall  with  over  50  per  cent,  opening,  and  a one-piece 
block  of  the  same  thickness  of  walls — 2 inches — would  also  be  lighter  to 
handle  and  doubtless  very  popular.  My  belief  is  that  the  one-piece 
block  of  the  future  will  be  24  inches  long  and  with  a thickness  of  walls 
of  not  over  2 inches.  Such  a block,  12  inches  wide  and  9 inches  high, 
will  weigh  only  97  lbs.,  and  if  well  and  honestly  made  will  bear  rough 
handling  and  any  possible  load. 

Finally,  it  is  to  the  appearance  of  concrete  blocks,  as  ordinarily 
made  and  used  that  architects  and  other  persons  of  taste  and  judgment 
make  the  greatest  objection.  Anything  that  savors  of  imitation  that 
pretends  to  be  what  it  is  not,  will  always  be  hated  and  condemned  by 
all  who  know  the  difference  between  the  good  and  the  bad.  The  com- 
mon rock-faced  block  is  an  imitation  of  the  cheapest  form  of  quarry 
stone  and  a poor  imitation  at  that,  for  no  two  natural  stone  blocks  are 
alike  in  surface;  while  even  if  you  have  half  a dozen  rock-face  plates 
of  the  same  size  of  block,  and  strive  to  shuffle  up  the  product  of  these 
plates  in  the  yard  and  on  the  work,  you  will  never  see  a building  in 
which,  here  and  there,  blocks  from  the  same  plate  are  not  found  one 
above  or  beside  the  other.  And  it  is  surprising  how  unerringly  the  eye 
will  pick  out  the  spots  where  this  occurs,  and  what  a feeling  of  “some- 
thing lacking”  is  awakened.  It  is  bad  art,  and  quite  indefensible.  The 
“rock-faced  galvanized  iron”  of  our  country  store-fronts  is  no  more 
a glaring  fraud.  The  rock-faced  block  must  go. 


26 


Now  let  us  inquire  what  constitutes  imitation,  and  how  concrete 
may  be  made  to  stand  on  its  merits  and  look  like  what  it  really  is.  In 
the  first  place,  concrete  must  always  look  like  stone,  because  it  is  stone. 
An  artificial  stone,  consisting  of  grains  of  sand  and  gravel  or  limestone 
crystals  bound  together  by  a little  Portland  cement,  cannot  help  look- 
ing like  natural  sandstone  or  limestone  made  up  of  the  same  materials 
bound  together  by  carbonate  of  lime  or  soluble  silicates  slowly  de- 
posited in  its  pores.  We  need  never  be  afraid  that  concrete  will  be  con- 
demned for  its  stony  look,  since  that  is  its  nature.  All  we  need  to  avoid 
ir  givng  the  work  an  appearance  which  is  unnatural  to  concrete,  such 
as  the  rock-face.  Smooth,  ribbed  and  paneled  surfaces,  also  good  or- 
namental patterns  for  friezes  or  cornices,  are  entirely  legitimate,  and 
equally  characteristic  of  stone,  metal,  terra  cotta  or  concrete.  Forms 
of  beauty  may  properly  be  reproduced  in  any  material;  the  only  thing 
to  be  avoided  is  pretense — the  attempt  to  deceive  the  observer  into 
the  belief  that  the  material  he  sees  is  something  different  from  what  it 
really  is. 

The  surface  which  best  pleases  the  eye  of  artist  and  architect  is  a 
rough  and  varied  one,  rather  than  the  smooth,  dead  look  which  rich 
cement  mixtures  have.  The  film  of  cement  which  coats  the  face  of 
the  work  is  certainly  monotonous  and  unattractive.  This  can  be  cheap- 
ly removed  by  washing  with  very  weak  acid,  and  very  beautiful  ef- 
fects are  thus  obtained,  especially  with  crushed  stone  or  gravels  con- 
taining pebbles  of  various  colors. 

CITY  SPECIFICATIONS  FOR  CONCRETE  BLOCKS. 

In  order  to  guard  against  the  use  of  blocks  of  poor  quality  and  to 
insure  safe  construction  of  block  buildings,  a number  of  cities  have 
adopted  specifications  for  the  acceptance  and  use  of  building  blocks 
of  concrete.  The  building  regulations  of  New  York  City*  in  regard 
to  all  materials  used  as  substitutes  for  brick  or  stone  are  extremely 
severe,  requiring  tests  to  be  made  on  blocks  the  size  and  shape  of  an 
ordinary  brick,  which  must  show  an  average  modulus  of  rupture  of  450 
lbs.  in  transverse  test,  average  compression  strength  of  3,000  lbs.,  water 
absorption  not  over  15  per  cent,  loss  of  not  more  than  33  per  cent 
strength  after  freezing  and  thawing  20  times,-  and  no  disintegration 
after  heating  1 hour  to  1,700  degrees  F.  and  plunging  into  cold  water. 

The  City  of  Philadelphia*  for  a time  followed  these  requirements, 
but  has  lately  modified  them,  and  provides  that  tests  of  hollow  con- 
crete blocks  shall  be  made  on  full-sized  specimens.  The  most  impor- 
tant requirements  are: 


27 


Rlocks  to  be  made  «of  Portland  cement  with  not  more  than  5 parts 
sand  and  gravel  or  crushed  rock;  hollow  space  to  be  not  over  33  per 
cent  (20  and  25  per  cent  in  lower  parts  of  high  walls);  maximum  load 
111  lbs.  per  square  inch  of  wall ;' crushing  strength  1,000  lbs.  per  square 
inch  of  total  surface  of  block  including  openings;  absorption,  freezing 
and  fire  tests  as*  in  New  York  requirements. 

According  to  the  Cement  Age,  concrete  blocks  in  the  Philadelphia 
market  have  shown  compression  strength  of  1,200  to  1,600  lbs.,  absorp- 
tion of  about  5 per  cent,  little  loss  of  strength  on  freezing,  and  have 
passed  the  fire  test  well. 

The  City  of  Newark,  N.  J.,  requires  that  blocks  shall  be  not  poorer 
than  1 to  4;  they  must  be  no  more  than  36  inches  long  and  10  inches 
high,  and  not  less  than  8 nor  more  than  16  inches  wide;  the  hollow 
spaces  must  not  exceed  one-third;  they  must  not  be  used  until  30  days 
old,  and  must  show  a crushing  strength  of  1,500  lbs.  per  square  inch. 

These  various  city  requirements  seem  generally  reasonable  and 
certainly  abundantly  severe.  It  is  difficult  to  see,  however,  why  the 
hollow  spaces  should  be  limited  to  one-third  or  less  when  strength  is 
fully  provided  for  by  a compression  requirement  of  1,000  lbs.  on  the 
whole  area  of  the  block.  If  blocks  with  thinner  walls  will  show  this 
strength,  there  appears  to  be  no  ground  for  prohibiting  them. 


THE  REINFORCED  CONCRETE  FACTORY  FOR  THE  AMERI- 
CAN OAK  LEATHER  CO.,  CINCINNATI. 

I 

At  least  seven  such  structures,  including  warehouses,  factories, 
office  buildings  and  stores,  were  then  in  process  of  erection.  The  ten- 
dency in  Cincinnati  recently  has  been  to  build  the  exterior  walls  of  hol- 
low blocks,  and  the  structural  portions  of  the  building,  that  is,  the  col- 
umns, beams  and  floors,  of  reinforced  concrete.  This  is  due  to  economi- 
cal considerations.  Because  of  the  cost  of  lumber  and  of  the  labor  of 
placing  and  removing  wall  forms  and  the  necessity  of  specially  treating 
the  face  of  the  concrete,  or  else  veneering  it  with  stone  or  brick,  it  is 
often  cheaper  to  build  the  entire  wall,  except  the  trimmings,  of.  hollow 
blocks. 

Among  the  pioneers  in  reinforced  concrete  construction  is  the  Ferro- 
concrete Construction  Co.,  the  builders  of  the  famous  sixteen-story  In- 
galls office  building  and  several  other  structures,  including  the  factory 
of  the  American  Oak  Leather  Co.,  also  in  Cincinnati. 


28 


The  building  is  designed  for  heavy  loading,  and  in  anticipation  of  the 
presence  of  piles  of  leather  on  all  or  nearly  all  the  floors  at  the  same 
time,  this  heavy  loading  was  carried  through  to  the  foundations.  The 
design  of  the  floor  plan  and  the  connection  of  the  building  with  an  old 


one  belonging  to  the  same  company  required  a large  variety  of  sizes  of 
floor  panels,  each  of  which  was  especially  designed  in  thickness  and  re 
inforcement  for  its  particular  load. 

The  building  is  seven  stories  high  above  the  basement,  and  the  base- 
ment floor  is  full  of  tanks  and  vats  and  troughs,  all  of  reinforced  con- 
crete, for  use  in  the  operations  incident  to  the  preparation  of  leather. 

Fig.  1 is  a typical  section  across  the  building.  The  main  portion  of 
the  structure  represented  by  the  three  bays  at  the  right  is  58  ft.  wide  by 
269  ft.  in  length.  Wings,  eight  stories  high  above  the  basement  and  pro- 
jecting at  each  end  of  the  building  carry  the  reinforced  concrete  stair- 
ways and  also  connect  with  the  old  building. 

The  columns  vary  in  size  in  accordance  with  the  spans  they  carry, 
ranging  from  10x10  in.  to  32x36  in.  The  principal  girders,  that  is,  the 
girders  across  the  building,  range  in  size  from  8x20  in.  to  14x20  in.  and 
10x24  in.  The  longitudinal  beams  which  butt  into  the  principal  girders 
range  from  6x16  in.  to  8x20  in.  The  floor  slabs  vary  with  the  span  from 
4 to  7 in.  in  total  thickness. 

For  a reinforced  concrete  building  nearly  as  many  carpenters  are 
required  as  laborers,  and  one  of  the  first  essentials  for  economical  con- 
struction is  the  design  of  the  forms  to  reduce  the  quantity  of  lumber  to  a 
minimum,  and  the  construction  of  these  forms  at  the  smallest  possible 
labor  cost.  In  the  present  case  one  of  the  first  operations  was  the  erec- 
tion of  a shanty  occupying  half  of  the  street  next  to  the  site,  which  was 
fortunately  on  an  unfrequented  highway,  and  equipping  it  with  power 
saws  and  other  woodworking  machinery.  Here  all  the  forms  required 
in  the  construction  of  the  building  were  made,  and  the  general  repairing 
was  done. 

Structural  Details. — Twisted  steel  was  used  for  reinforcement.  The 
square  rods  were  twisted,  cut  to  length,  and  bent  to  shape  at  the  per- 
manent shop  of  the  contractors  in  the  city.  The  twisting  machine  twists 
three  “30-ft.  rods  of  the  smaller  sizes  at  the  same  operation.  As  the  opera- 
tion of  twisting  a set  of  three  rods  occupies  two  men  with  the  machine 
but  slightly  over  one  minute  (not  including  the  carrying  to  and  from  the 
machine),  the  cost  is  scarcely  appreciable,  while  the  twisting  produces  a 
deformed  rod  capable  of  greater  adhesion  and  with  an  increased  elastic 
limit.  The  high  elastic  limit  was  utilized  by  the  designers  in  the  as- 
sumption of  a higher  allowable  unit  pull  in  the  steel  and  thus  a smaller 
percentage  of  the  metal  in  the  beams  and  slabs. 


30 


(By  practical  experience  it  has  been  shown  that  a twisted  rod  is 
weaker  than  an  untwisted  rod;  by  twisting,  the  rod  is  deformed  and  nat- 
urally weaker.) 

A cutter  designed  with  a multiple  lever,  so  as  to  be  operated  by  one 
man,  cuts  single  rods  up  to  y%  in.  square,  and  smaller  rods  in  lots  of  two 
or  more. 

All  rods  are  bent  cold.  The  small  rods  up  to  about  Y in.  square, 
which  comprise  all  the  steel  which  requires  bending  except  the  bent  bars 
in  the  girders,  are  bent  by  hand  with  the  aid  of  a special  vise.  For  rods 
larger  than  y in.  a machine  designed  for  the  purpose  bends  the  rod  to 
any  angle  and  at  the  same  time  keeps  all  the  bends  in  the  same  rod  in  a 
plane. 

As  usual  in  concrete  building  construction,  the  concrete  was  mixed  on 
the  ground  and  elevated  to  the  floor  where  it  was  required.  However, 
instead  of  following  the  more  common  practice  of  an  elevator  running 
in  a frame  which  is  raised  from  story  to  story  as  the  building  advances, 
an  immense  derrick  with  an  80-ft.  boom  was  set  on  top  of  a tower  con- 
sisting of  a pyramidal  frame  of  timber  with  its  diagonal  braces  carefully 
bolted.  The  base  of  the  derrick  was  thus  55  ft.  above  the  ground  and 
so  high  that  buckets  could  be  emptied  upon  the  roof.  This  derrick  was 
used  not  only  for  hoisting  the  concrete,  but  for  raising  the  form  timber 
and  handling  other  material  and  tools. 

It  was  thought  when  the  building  was  begun  that  it  would  be  the 
best  plan  to  dump  the  concrete  from  the  bucket  at  various  places  on  the 
floor  where  it  was  required,  the  boom  being  long  enough  to  swing  over 
a considerable  area  of  the  floor.  This  worked  well  in  the  lower  stories, 
but  for  the  upper  floors  and  the  roof,  where  the  swing  of  the  boom  be- 
came limited,  it  was  found  more  economical  to  dump  the  concrete  into  a 
hopper  to  be  wheeled  in  barrows  to  place.  Fig.  2,  which  is  taken  on  the 
fifth  floor,  shows  the  operation  of  dumping  the  hoisting  bucket  into  the 
hopper.  By  this  plan  less  time  was  consumed  in  placing  the  bucket,  and 
no  tag-rope  man  was  required,  as  the  engine-man  could  swing  the  boom 
to  a certain  point  on  the  wall  which  brought  the  bucket  directly  over 
the  hopper. 

The  concrete  is  composed  of  Portland  cement,  sand  and  broken 
stone  in  proportions  1:2:4.  The  sand  and  broken  stone  were  stored  in 
bins  within  less  than  50  ft.  of  the  mixer,  and  wheeled  in  barrows,  which 
were  also  used  for  measuring,  along  an  elevated  run  to  a mixer.  From 
the  mixer  the  concrete  fell  into  the  derrick  bucket  which  rested  on  an 
iron  truck  on  wheels,  about  twice  the  length  of  the  bucket,  so  that  the 


31 


empty  bucket  could  be  set  by  the  derrick  on  one  end  of  the  truck  while 
the  other  bucket  was  being  filled,  and  then  as.  soon  as  the  full  bucket 
was  removed,  the  truck  was  pushed  by  the  attendant  to  bring  the  empty 
bucket  under  the  mixer. 

The  steel  in  the  columns,  consisting  generally  of  vertical  round  rods 
with  hoops  placed  around  them  every  foot  in  height,  was  set  as  soon  as 
the  concrete  of  any  floor  was  laid,  the  column  forms  were  built  around  it, 
and  the  floor  slab  and  girder  forms  placed  and  carefully  supported  and 
braced  by  vertical  struts  and  diagonals.  The  forms  were  thus  built  and 
the  steel  placed  in  the  beams  and  slabs  so  that  the  concrete  was  poured 
in  one-half  of  the  floor  while  the  forms  were  being  built  for  the  other 
half  of  the  story. 

The  concrete  was  mixed  wet  enough  to  pour  into  the  columns  and  a 
very  fine  face  was  obtained  on  the  sides  of  the  posts  by  the  use  of  long- 
handled  wooden  paddles.  The  thickness  of  the  floor  slabs  were  gauged 
by  1x2  in.  wood  strips  with  blocks  nailed  on  the  under  side  of  them  at 
occasional  intervals  to  bring  the  top  of  the  strip  to  the  required  surface 
level.  These  were  placed  crosswise  of  the  floor  about  every  15  ft.  and 
the  concrete  poured  between  them,  and  screened  with  a long  straight 
edge.  The  strips  were  immediately  removed,  and  their  location  filled 
with  concrete  by  men  wearing  rubber  boots  who  walked  through  the 
soft  material.  As  soon  as  the  concrete  was  sufficiently  set,  the  surface 


32 


finish  was  spread  and  finally  floated. 

The  rate  of  speed  on  the  building  was  a half  story  per  week. 
Design. — The  general  floor  plan  is  shown  in  Fig.  3,  which  shows 
the  forms  and  the  steel  in  place  for  the  50-ft.  span  girders  and  the  floor' 


TYPE  M 


TYPE  N 


Fi g.  6 

in  the  west  end  of  the  building.  These  girders  are  14  in.  wide  and  36  in. 
deep,  and  the  span  is  50  ft.  in  the  clear. 


33 


Fig.  3 


Fig.  8 shows  girder  and  floor  slab  construction. 

Fig.  4 shows  how  stirrups  are  placed  on  girder  rods.  The  surfaces 


W Hoops  12"  0.0 


h o) 


COL.  33 


34 


of  the  girders  and  floors  show  the  knots  and  other  impressions  from  the 
lumber  of  the  forms,  but  a close  examination  fails  to  detect  any  of  the 
irregularities  and  stone  pockets  so  often  found  in  structures  of  this  char- 
acter. 

In  general,  there  were  four  principal  rods  in  each  beam  and  two  of 
these  were  bent  up  diagonally  so  as  to  reach  the  top  of  the  beam  or  to 
extend  over  the  supports.  Typical  forms  of  these  bent  bars  are  shown 
in  Fig.  6. 

The  live  loads  assumed  in  the  design  are  as  follows:  First  floor, 
lower  section,  500  lbs.  per  square  foot;  floors  over  50-ft.  span  150  lbs.; 
other  floors,  200  lbs.;  roof,  100  lbs.  The  girders  are  calculated  fur  80 
per  cent  of  the  live  load.  The  columns  take  the  total  dead  load,  and 
also  are  assumed  to  carry  the  following  percentages  of  the  live  load  com- 
ing from  the  girders:  On  the  roof,  100  per  cent;  seventh  floor,  100  per 

cent;  sixth  floor,  90  per  cent;  fifth  floor,  80  per  cent;  fourth  floor,  70  per 
cent;  thi-rd  floor,  60  per  cent;  second  floor,  50  per  cent;  first  floor,  50 
per  cent. 

Typical  column  reinforcement  adapted  for  different  sectional  dimen- 
sions is  shown  in  Fig.  7.  In  general,  the  vertical  rods,  which  are  round, 
have  a sectional  area  of  about  1 to  2 per  cent  of  the  cross  section  of  the 
column.  The  size  of  the  rods  is  reduced  from  story  to  story,  ranging  on 
an  average  from  about  2 in.  in  the  lower  floors  to  *4  in.  in  the  upper 
stories. 

The  foundations  for  the  columns  are  reinforced,  and  as  they  are  built 
in  advance  of  the  columns,  short  vertical  rods  about  3 ft.  long  are  set  into 
them,  which  project  up  about  2 ft.  into  the  column.  The  lower  ends  of 
these  “stubs”  are  set  upon  plates  which  hold  them  in  position,  and  form 
a bearing  upon  the  concrete. 

For  the  columns  at  one  end  of  the  building  an  inverted  beam  foun- 
dation was  required  because  the  footings  could  not  project  beyond  the 
building  line.  This  foundation,  supporting  two  polumns,  was  heavily  re- 
inforced at  the  top  with  sixteen  24-in.  and  1-in.  rods,  and  provided  with 
stirrups  as  in  ordinary  beam  construction. 

Note. — The  contractors  collect  in  one  table  and  the  above  diagram 
all  the  data  for  the  reinforcement  of  all  beams  and  girders.  For  example, 
the  beam  a is  shown  in  this  table  to  have  bent  reinforcement  of  type  R, 
of  the  dimensions  tabulated,  as  well  as  straight  bars  and  stirrups.  This 
tabulation  and  diagram  keep  the  floor  plans  free  from  lettering  about 
details. 

The  stairs  are  reinforced  as  shown  in  Fig.  9,  which  run  up  in  the 
wings  at  each  end  of  the  building  are  generally  in  double  flights  with 


35 


winders  or  platforms  connecting  them.  They  are  usually  enclosed  on 
three  sides  by  the  brick  wall  of  the  building.  The  plan  and  elevation  of 
one  of  the  flights  with  winders  is  shown  in  Fig.  1.  In  the  more  usual 


pattern,  with  a platform  at  the  half-story,  no  post  is  required. 

A large  portion  of  the  basement  is  occupied  with  tanks  or  vats  for 

use  in  the  leather  processes.  These  pickling  vats  are  each  8 ft.  long,  6 

ft.  deep,  with  walls  only  2x/2  in.  thick,  constructed  all  of  concrete. 

The  plan  of  the  vats  in  three  of  the  bays  running  across  the  build- 
ing, which  also  gives  a section  of  a vat  wall  and  of  one  of  the  drains 

below  the  vats.  The  vats  are  built  in  groups,  there  being  a group  of  six 
vats  in  each  bay.  Each  group  is  built  at  one  operation,  so  that  there 
will  be  no  joint,  and  the  exterior  walls  of  the  group,  that  is,  the  walls 
on  a line  with  the  columns  both  ways,  are  thus  double,  but  separated  so 
as  to  permit  shrinkage.  The  bottom  of  the  vats  are  about  6 ft.  above 
the  ground,  and  they  are  supported  by  small  columns  at  each  intersec- 
tion. 

The  troughs  or  drains  which  run  under  the  vats  are  of  unique  con- 
struction. They  are  built  as  V-shaped  troughs  of  mortar,  2 in.  thick 
and  8 in.  deep,  reinforced  with  J^-in.  longitudinal  rods  and  also  with 
14-in.  hoops  which  are  allowed  to  project  about  2 ft.  above  the  sides 
when  the  trough  is  finished.  The  floors  of  the  vats  constitute  the  tops 
of  these  troughs.  In  some  cases  a thin  oak  veneer  is  sprung  across  the 
top  of  the  drain  to  serve  as  a form  for  the  concrete,  and  the  floors  of  the 
vats  are  then  spread  over  this  and  over  the  surface  of  floor  between  the 
drains,  and  the  ends  of  the  hoops  which  project  above  the  sides 

of  the  drain  are  bent  down  into  this  floor  so  that  the  drains  are  in  effect 
suspended  from  the  floor  and  form  a part  of  it. 

As  is  stated  above,  the  walls  of  the  building  are  of  brick  (hollow  blocks 


36 


would  have  lessened  the  cost  of  the  building  10  per  cent),  but  the  water 
table,  the  sills  and  the  caps  of  the  windows  and  doors  are  formed  o* 
concrete  laid  in  place.  No  mortar  surface  is  given  to  them,  but  the  ag- 
gregate is  sand  and  rather  fine  broken  stone,  and  care  is  used  in  placing 
it  against  the  form. 

After  removing  the  forms,  the  surfaces  are  carefully  dressed  by  pol- 
ishing with  a piece  of  sandstone  so  that  they  can  scarcely  be  distinguished 
from  cut  stone  trimmings. 

SPECIAL  FALSEWORK  FOR  A CONCRETE  BRIDGE. 

The  Cobbs  Creek  bridge  at  Media,  Pa.,  near  Philadelphia,  carries  a 
double  track  electric  railway  and  a highway  about  19  ft.  above  the  creek 
level  by  a single  skew  span  46  ft.  2 in.  in  the  clear.  The  arch  is  a false 
ellipse  of  reinforced  concrete.  The  intrados  is  a three-centered  curve 
having  a total  rise  of  17  ft.  5 in.  from  skewback  to  crown.  The  central 
part  has  a radius  of  30  ft.  8 in.,  forming  a segment  subtended  by  a chord 
30^2  ft.  long.  At  each  end  this  segment  is  tangent  to  a segment  of  15 
ft.  4*4  in.  radius,  which  is  continued  beyond  the  springing  fine  nearly 
to  the  intersection  with  the  inclined  plane  of  the  footing.  The  extrados 
is  of  similar  construction  with  center  and  end  radii  of  40  ft.  and  16  ft. 
10  in.  respectively.  The  end  segments  do  not,  however,  extend  quite  to 
the  springing  line  but  just  above  that  point  are  tangent  to  reversed 
curves  which  diverge  from  the  intrados  so  as  to  give  the  footings  a 
width  of  6 ft.  8 in.  in  a plane  perpendicular  to  the  direction  of  the  re- 


Fig.  13 

sultant.  The  radial  thickness  of  the  arch  ring  varies  from  10  in.  at  the 
crown  to  18  in.  at  the  extremities  of  the  center  segments  and  to  about 
4 ft.  at  the  springing  line. 


37 


The  reinforcement  is  made  wholly  of  plain  round  bars  of  medium 
steel  in  two  sets,  one  of  them  near  the  upper  and  the  other  near  the 
lower  surface  of  the  concrete.  In  the  lower  set  there  are  eighty  curved 
rods  \yA  in.  in  diameter  and  30  ft.  long  which  reach  from  the  footing  to 
a point  near  the  crown  where  they  overlap  40  rods  of  the  same  diameter 
24  ft.  long.  The  upper  bars  correspond  to  them  in  relative  positron  and 
number,  but  are  only  in.  in  diameter  at  the  crown,  those  at  the  end 
being  1^-in.  rods.  The  rods  in  both  sets  are  crossed  by  horizontal  *4- in. 
rods  parallel  to  the  axis  of  the  arch  and  spaced  15  in.  apart  on  centers. 
Each  footing  is  reinforced  by  forty  ^-in.  rods  6 *4  ft.  long  and  12  in. 
apart,  perpendicular  to  the  resultant.  These  are  crossed  by  two  full- 
length  in.  horizontal  transverse  rods  at  the  extremities  of  the  curved 
rods  in  the  upper  and  outer  sets  of  the  arch  ring.  The  arch  is  40  ft. 
long  inside  the  parapet  walls,  and  its  axis  is  inclined  74  deg.  with  that 
of  the  roadway.  At  each  end  the  spandral  walls,  which  are  not  shown 
in  the  accompanying  photograph,  are  continuous  with  the  intersecting 
wing  walls,  the  latter  being  oblique  both  to  the  axis  of  the  street  and 
to  the  axis  of  the  stream.  The  bridge  contains  about  200  cu.  yd.  of  1:3:5 
concrete  mixed  wet  by  hand,  Vulcanite  Portland  cement  a;id  broken 
stone  varying  from  54  to  ^4  in-  in  diameter  being  used.  There  are 
about  30  tons  of  steel  in  the  reinforcement  and  the  arch  was  built  in 
ten  working  days  after  the  completion  of  the  falsework. 

The  stream  was  considered  treacherous  and  subject  to  unexpected 
floods,  and  as  the  contractors  did  not  know  the  character  of  the  bottom 
it  was  decided  not  to  trust  falsework  bents  placed  in  the  bed  of  the 
stream.  The  locality  was  not  such  that  it  was  convenient  or  economical 
to  drive  piles,  and  it  was  therefore  decided  to  support  the  bridge  during 
construction  on  falsework  trusses,  as  shown  in  the  illustration.  Sheeted 
pits  were  excavated  to  rock  at  a depth  of  8 to  10  ft.  below  the  surface 
of  the  ground  and  from  6 ft.  below  water  level.  These  were  drained 
without  serious  difficulty  by  a centrifugal  pump  and  the  lower  ends  of 
the  arch  ring,  which  really  acted  as  skewback  piers,  were  built  in  them 
about  up  to  the  springing  line.  They  were  allowed  to  set  and  serve  as 
supports  for  the  falsework,  which  was  made  of  simple  wooden  trusses 
about  3 ft.  apart.  Each  truss  was  built  of  3xl0-in.  planks  spiked  together 
at  intersections  and  serving  to  support  the  top  chord;  which  was  com- 
posed of  scarf  boards  carefully  cut  to  the  curve  of  the  intrados.  The 
trusses  were  built  in  place  and  the  light  radial  members  seen  in  them 
are  small  strips  of  wood  put  in  place  before  the  completion  of  the  truss 
to  support  the  scarf  boards  temporarily  until  the  connections  were  com- 
pleted and  the  structural  developed  full  strength.  The  trusses  were 


38 


braced  together  by  intermediate  horizontal  planks  and  by  the  lagging, 
which  consisted  of  square-edge  boards  planed  on  the  upper  surface. 
Outside  forms  were  built  in  three  sections  at  each  end  of  the  arch  and 
the  concrete  was  rammed  in  them  to  points  above  the  haunches,  as  in- 
dicated by  the  angles  of  the  extrados  between  the  face  rings.  By  dar- 
ing out  the  wedges  under  the  falsework,  the  latter  was  released  three 
weeks  after  the  concrete  was  laid,  without  causing  any  appreciable  set- 
tlement in  the  arch. 

A FLAT  SPAN  REINFORCED  CONCRETE  BRIDGE  AT  MEMPHIS 

• 

A 100-ft.  through  span,  reinforced-concrete  highway  bridge,  carried 
by  two  longitudinal  girders,  without  any  dependence  on  arch  action,  has 
recently  been  built  at  Memphis,  Tenn.  The  bridge  replaces  an  old  wood- 
en structure  and  spans  a 100-ft.  right-of-way  on  which  six  tracks  of 
lour  different  railroads  are  laid.  The  railroad  tracks  closely  parallel 
one  side  of  a "cemtery  so  that  the  approach  to  one  end  of  the  bridge  is 
within  the  grounds  of  the  latter,  the  level  of  the  grounds  being  about 
15  ft.  above  the  tracks.  Owing  to  the  necessity  of  providing  a clearance 
of  at  least  19  ft.  over  all  of  the  tracks  on  the  right-of-way,  and  to  very 
strong  objections  to  a slightly  graded  approach  that  would  have  been 
required  in  the  cemetery  grounds  if  an  arched  bridge  which  would  pro1- 
vide  such  a clearance  had  been  built,  a practically  flat  span  had  to  be  de- 
signed and  has  been  erected. 


39 


rise  is  largely  introduced  near  the  abutments.  The  two  longitudinal 
girders  are,  designed  to  carry  the  floor,  which  is  also  reinforced  and  is 
suspended  from  them.  Each  end  of  both  girders  is  designed  to  act  as 
a cantilever  beam  for  21  ft.  from  the  abutments.  Between  the  outer 
ends  of  the  two  cantilever  parts  of  each  girder  the  latter  is  designed  to 
act  as  a simple  beam,  with  a span  of  58  ft.  carried  by  the  cantilevers. 

The  bridge  has  a total  width  of  31  ft.  made  up  of  a 16-ft.  roadway, 
with  a walk  on  each  side,  one  girder  being  between  the  roadway  and 
each  walk.  The  girders  are  3 ft.  6 in.  wide  and  have  a total  height  of 
6 ft.  6 in.,  including  a 6-in.  coping.  The  girders  being  designed  to  act 
as  Cantilevers  toward  the  abutments,  tension  stresses  are  produced  near 
their  upper  surfaces  in  this  portion  of  them  and  the  concrete  has  been 
reinforced  accordingly.  This  reinforcement  consists  of  thirty  lj4-in. 
bars,  placed  in  four  horizontal  rows  as  shown  in  one  of  the  accompany-’ 
ing  illustrations.  These  bars  are  40  ft.  long,  extending  4 ft.  beyond  the 
outer  end  of  the  cantilever  section  of  the  girder  and  15  ft.  back  into  the 
e nd  of  the  girder  over  the  abutment.  The  simple  beam  portion  of  each 
girder  is  reinforced  near  the  bottom  by  twenty-four  1^-in.  bars  in  three 
rows.  This  reinforcement  is  66  f»t.  long  and  extends  4 ft.  into  the  outer 
end  of  each  of  the  cantilever  sections  supporting  the  simple  beam.  Two 
planes  of  no  moment  occur  in  each  girder,  one  at  the  junction  between 
the  end  of  each  cantilever  and  the  adjoining  end  of  the  simple  beam. 
Heavy  shearing  stresses  are,  therefore,  introduced  at  these  points.  These 
stresses  are  overcome  by  thirty  short,  inclined  1^2-in.  bars  placed  in  the 
vicinity  of  each  of  the  planes  of  no  moment. 

Each  cantilever  is  anchored  to  its  abutment  by  a cluster  of  lj/2-in. 
bars,  which  extend  from  a central  point  near  the  top  of  the  girder  and 
over  the  haunch  of  the  arch  down  into  the  concrete  of  the  abutment,  ra- 
diating in  the  latter  to  secure  more  complete  anchorage.  Several  light 
street  railway  rails  were  also  placed  in  each  girder  back  of  the  point  of 
support  to  bond  the  masonry  of  the  girder  and  the  abutment  together. 

The  abutments  had  to  be  designed  to  provide  anchorages  for  the 
cantilevers,  the  thrust  from  the  arch  being  considered  of  little  conse- 
quence. The  section  of  the  girders  is  extended  well  back  from  the  point 
of  support  of  the  cantilevers  and  down  to  the  bottom  of  the  abutments. 
Between  the  girders  the  abutments  are  hollow,  having  a retailing  wall 
at  the  front,  with  a wide,  flat  floor  cantilevered  back  from  the  rear  face 
of  the  wall.  This  floor  is  reinforced  near  the  lower  surface  with  light 
rails,  the  ends  of  which  extend  into  the  girder  abutments.  The  front 
wall  thus  retains  an  earth  load  on  the  floor  and  provides  an  additional 
anchorage. 


40 


The  bridge  floor  is  13  in.  thick  and  is  reinforced  with  I-beams  placed 
transversely.  Each  beam  is  attached  on  the  center  line  of  both  bridge  gird- 
ers to  two  1-in.  tie  rods.  These  rods  extend  up  into  the  girder  and  are 
anchored  just  below  the  coping  on  the  latter. to  a longitudinal  y2x6- in. 
steel  bar  imbedded  in  the  concrete.  The  floor  is  thus  suspended  from  the 
girders  and  is  not  designed  to  be  self-supporting  due  to  any  arch  action 
that  may  occur  in  it.  It  is  designed  to  carry  a uniform  live  load  of  200 
lb.  per  square  foot.  This  load  is  small,  however,  as  compared  with  the 


dead  load  of  the  bridge,  amounting  to  3 cu.  yd.  of  concrete  per  linear  foot 
of  span,  which  the  girders  are  required  to  carry. 


41 


The  erection  of  the  forms  for  the  span  was  rendered  difficult  and  ex- 
pensive by  the  necessity  of  maintaining  traffic  on  the  six  railroad  tracks. 
The  dead  load  of  the  bridge  to  be  carried  by  these  forms  was  approxi- 
mately 1,000,000  lb.  Spans  of  as  much  as  20  ft.  were  required  in  the 
falsework  for  the  centering  in  order  to  clear  the  tracks.  The  bents  of 
the  forms  were  built  of  12xl2-in.  timbers,  heavily  X-braced.  Resting  on 
the  caps  of  these  bents  were  5xl6-in.  stringers,  placed  clo-sely  together, 
which  carried  the  lagging. 

The  concrete  was  mixed  fairly  wet  in  the  proportions  of  1 of  Port- 
land cement,  2l/2  sand  and  5 of  small  broken  stone.  Corrugated  bars 
were  used  for  reinforcement,  except  as  mentioned.  One  girder  was  built 
in  a single  day  and  the  other  the  following  day,  each  of  the  girders  be- 
ing considered  as  a separate  structure  during  the  construcion.  After  the 
concrete  had  set  three  months  the  forms  were  removed  without  any  set- 
tlement occurring,  insofar  as  it  was  possible  to  discover.  No  cracks 
are  apparent  in  the  structure,  except  a very  few  small  ones  in  the  coping, 
which  is  not  reinforced  and  has  no  expansion  joints. 

The  bridge  cost  $17,500  complete,  including  the  asphalt  pavement  on 
the  roadway,  a cut-stone  veneer  on  the  posts  at  the  ends  of  the  girders 
and  the  iron  railing  along  the  walk  on  each  side.  The  forms  required 
$4,000  of  this  amount,  owing  chiefly  to  the  difficulty  of  erecting  them 
so  they  would  not  interfere  with  traffic  on  the  tracks.  The  girders  and 
the  floor  of  the  span  contain  about  200  and  the  abutments  800  cu.  yd. 
of  concrete.  The  latter  quantity  could  be  greatly  reduced  in  proportion 
to  the  former  in  a structure  with  several  spans  of  the  same  type,  since 
the  reinforcement  at  the  top  of  the  cantilever  sections  of  the  girders  could 
be  extended  through  adjoining  spans  and  the  heavy  anchorage  required 
in  a structure  with  a single  span  avoided. 


42 


Method  of  Facing  Concrete  Work  (this  Plate  is  made  of  Sheet  Iron) 


Barn 


43 


44 


Colored  Hlortars 


Colors  given  to  Portland  Cement  Mortars,  containing  two  parts  sand  to 
one  cement.  Remember  that  wet  mortars  give  a darker  color  but 
dries  out  lighter. 

White  Cement  Blocks  are  made  by  using  lime-stone  siftings.  White  sand 
or  Marble  dust  for  the  facing. 


Weight  of  Coloring  Matter  to  100  pounds  of  Cement 

Dry  Material 

Used 

' 

-1 

3/5  Pound 

1 l/5  pounds 

1 4/5  pounds 

4/5  Pound 

Lamp  Black 

Light  Slate 

Light  Gray 

Blue  Gray 

Dark  Blue 
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Prussian  Blue 

Light  Green 
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Blue  Slate 

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Light  Terra 
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Light  Brick 
Red 

Red  Iron  Ore 

Pinkish  Slate 

Dull  Pink 

Terra  Cotta 

Light  Brick 
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45 


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47 


.SAINT 


OHLENDORF 

LOUIS 


PRINT 


' 


